The Brass Plates were an important resource for Lehi, Nephi, and the generations that followed. So it is important to have a basic understanding of of them and their history. They were apparently begun by Joseph the son of Jacob while he resided in Egypt, and contained some of his writings and history. Later writers added the books of Moses which included an account of the creation and the subsequent history through the exodus. Subsequent additions included the histories of the kings and prophets down through Isaiah and Jeremiah.
The plates also included other writings not found in our Bible. Prophets such as Zenos, Neum and Zenock whose teachings are quoted in the Book of Mormon. They also contain some of the writings and history of Joseph son of Jacob which was never available to the biblical scribes.
The Brass Plates were written in the Egyptian script which was used by the Jews to write Hebrew, much as we used the Arabic alphabet to write English. It was an abbreviated script which was easier to write than was the Hebrew. It was something that had to be learned much as shorthand is learned in our day. Apparently Lehi knew how to read and write the script, and taught Nephi to use it as well (see 1 Ne. 1:1.) It does not appear that Laman and Lemuel had learned the Egyptian script as Lehi and Nephi had to read and explain the writings on the plates to them.
This Egyptian script was used by Nephi and the subsequent Nephite prophets to record their prophecies, teachings and histories over the years. It eventually evolved into the "reformed Egyptian" of the Gold Plates (see Mosiah 1:3-4).
At the time of Lehi the Brass Plates were in the possession of Laban who was apparently a Jewish leader and a military official with authority under the king. He was of the linage of Joseph so had legal right to the plates. He was wealthy and had servants and soldiers at his command. He was unprincipled and likely used his position to his advantage. We are not told how long he had possessed the plates, but he apparently valued them, if not for their spiritual value, at least as an heirloom and kept them with his treasures.
The plates had been kept up to date by various unknown scribes and were current at the time of Lehi, including recent teachings and prophecies of Jeremiah. It seems ironic that Laban would have included the writings of Jeremiah as he probably didn't believe any of his prophecies, and may have actually been involved in the persecution of the prophet. Jeremiah was highly unpopular among the political leaders of Jerusalem and was viewed as a traitor as he favored the Babylonians. But whether Laban caused Jeremiah's writings to be included or not, they were there. Perhaps Laban had just recently come into possession of the plates, and they had earlier been kept by someone who believed Jeremiah. It is likely that Laban read and wrote the Egyptian script, and may have even studied the writings on the Brass Plates.
As a result of Laban's deceit and dishonesty it became necessary for the Lord to deliver him into Nephi's hands to be slain. It is obvious that Nephi could not have obtained the plates in any other way, and it was vitally important for Lehi's group to have them. So Laban was killed in the dead of the night and Nephi and his brothers fled with the plates and with Laban's servant. It was important for them to take Zoram with them as he could have notified the authorities of the death and they would have surely pursued them even down into the interior of Saudi Arabia. The prophet Urijah had been pursued down into Egypt and brought back and killed for a much minor offense.
Once the brothers return to their family, who were camped on the shore of the Red Sea, Lehi read the records (in the Egyptian script) and found that he was a descendant of Joseph. Apparently he had not known this before. He prophesied that the Brass Plates should never perish (this requires a miraculous preservation as brass would normally corrode over time), and that they should eventually go forth to all nations and peoples who would be his descendants (this will be a future event). Over time Lehi's descendants were taught the gospel from the records contained on the plates. Nephi took the plates with him when he and his people separated from his brothers. This caused his brothers and their posterity to hate him and his descendants for hundreds of years. The plates were kept by the Nephites as a national treasure (along with the sword of Laban and the Liahona) and handed down from one prophet/leader to another until they were buried by Mormon with all the other accumulated records in the original Hill Cumorah.
1 Ne. 5:10 Lehi, took the records which were engraven upon the plates of brass, and he did search them from the beginning.
11 And he beheld that they did contain the five books of Moses, which gave an account of the creation of the world, and also of Adam and Eve, who were our first parents;
12 And also a record of the Jews from the beginning, even down to the commencement of the reign of Zedekiah, king of Judah;
13 And also the prophecies of the holy prophets, from the beginning, even down to the commencement of the reign of Zedekiah; and also many prophecies which have been spoken by the mouth of Jeremiah.
14 And it came to pass that my father, Lehi, also found upon the plates of brass a genealogy of his fathers;
18 That these plates of brass should go forth unto all nations, kindreds, tongues, and people who were of his seed.
19 Wherefore, he said that these plates of brass should never perish; neither should they be dimmed any more by time.
1 Ne. 13:23 The book that thou beholdest is a record of the Jews, which contains the covenants of the Lord, which he hath made unto the house of Israel; and it also containeth many of the prophecies of the holy prophets; and it is a record like unto the engravings which are upon the plates of brass, save there are not so many; nevertheless, they contain the covenants of the Lord, which he hath made unto the house of Israel; wherefore, they are of great worth unto the Gentiles.
1 Ne. 19:21 And he surely did show unto the prophets of old all things concerning them; and also he did show unto many concerning us; wherefore, it must needs be that we know concerning them for they are written upon the plates of brass.
22 Now it came to pass that I, Nephi, did teach my brethren these things; and it came to pass that I did read many things to them, which were engraven upon the plates of brass, that they might know concerning the doings of the Lord in other lands, among people of old.
23 And I did read many things unto them which were written in the books of Moses; but that I might more fully persuade them to believe in the Lord their Redeemer I did read unto them that which was written by the prophet Isaiah; for I did liken all scriptures unto us, that it might be for our profit and learning.
1 Ne. 22:1 AND now it came to pass that after I, Nephi, had read these things which were engraven upon the plates of brass, my brethren came unto me and said unto me: What meaneth these things which ye have read? Behold, are they to be understood according to things which are spiritual, which shall come to pass according to the spirit and not the flesh?
2 Ne. 4:1 AND now, I, Nephi, speak concerning the prophecies of which my father hath spoken, concerning Joseph, who was carried into Egypt.
2 For behold, he truly prophesied concerning all his seed. And the prophecies which he wrote, there are not many greater. And he prophesied concerning us, and our future generations; and they are written upon the plates of brass.
2 Ne. 5:12 And I, Nephi, had also brought the records which were engraven upon the plates of brass; and also the ball, or compass, which was prepared for my father by the hand of the Lord, according to that which is written.
Omni 1:14 And they discovered a people, who were called the people of Zarahemla. Now, there was great rejoicing among the people of Zarahemla; and also Zarahemla did rejoice exceedingly, because the Lord had sent the people of Mosiah with the plates of brass which contained the record of the Jews.
Mosiah 1:3 And he also taught them concerning the records which were engraven on the plates of brass, saying: My sons, I would that ye should remember that were it not for these plates, which contain these records and these commandments, we must have suffered in ignorance, even at this present time, not knowing the mysteries of God.
4 For it were not possible that our father, Lehi, could have remembered all these things, to have taught them to his children, except it were for the help of these plates; for he having been taught in the language of the Egyptians therefore he could read these engravings, and teach them to his children, that thereby they could teach them to their children, and so fulfilling the commandments of God, even down to this present time.
Mosiah 10:16 And again, they were wroth with him because he departed into the wilderness as the Lord had commanded him, and took the records which were engraven on the plates of brass, for they said that he robbed them.
17 And thus they have taught their children that they should hate them, and that they should murder them, and that they should rob and plunder them, and do all they could to destroy them; therefore they have an eternal hatred towards the children of Nephi.
Alma 37:3 And these plates of brass, which contain these engravings, which have the records of the holy scriptures upon them, which have the genealogy of our forefathers, even from the beginning—
4 Behold, it has been prophesied by our fathers, that they should be kept and handed down from one generation to another, and be kept and preserved by the hand of the Lord until they should go forth unto every nation, kindred, tongue, and people, that they shall know of the mysteries contained thereon.
5 And now behold, if they are kept they must retain their brightness; yea, and they will retain their brightness; yea, and also shall all the plates which do contain that which is holy writ.
6 Now ye may suppose that this is foolishness in me; but behold I say unto you, that by small and simple things are great things brought to pass; and small means in many instances doth confound the wise.
7 And the Lord God doth work by means to bring about his great and eternal purposes; and by very small means the Lord doth confound the wise and bringeth about the salvation of many souls.
8 And now, it has hitherto been wisdom in God that these things should be preserved; for behold, they have enlarged the memory of this people, yea, and convinced many of the error of their ways, and brought them to the knowledge of their God unto the salvation of their souls.
9 Yea, I say unto you, were it not for these things that these records do contain, which are on these plates, Ammon and his brethren could not have convinced so many thousands of the Lamanites of the incorrect tradition of their fathers; yea, these records and their words brought them unto repentance; that is, they brought them to the knowledge of the Lord their God, and to rejoice in Jesus Christ their Redeemer.
10 And who knoweth but what they will be the means of bringing many thousands of them, yea, and also many thousands of our stiffnecked brethren, the Nephites, who are now hardening their hearts in sin and iniquities, to the knowledge of their Redeemer?
11 Now these mysteries are not yet fully made known unto me; therefore I shall forbear.
12 And it may suffice if I only say they are preserved for a wise purpose, which purpose is known unto God; for he doth counsel in wisdom over all his works, and his paths are straight, and his course is cone eternal round.