The Brass Plates were inscribed with ancient scriptures, many of them corresponding with books from the Old Testament, that the prophet Lehi and his family took with them to the promised land of the Americas.
These plates were in the possession of Laban, a sage and elder in Jerusalem. Nephi explains that “Laban also was a descendant of Joseph, wherefore he and his fathers had kept the records” (1 Nephi 5:16.) Though Lehi, as a prophet warning Judah before the Babylonian captivity, was warned to flee into the wilderness with his family for safety, the Lord then commanded Lehi's sons to return to Jerusalem to obtain the plates.
- Although the information is sparse concerning the origin of the plates of brass, the Book of Mormon is quite detailed on what the plates contained. According to Nephi, the plates of brass contained the books of Moses and the Mosaic law (see 1 Nephi 4:15–16; 1 Nephi 5:11), tying the Nephites to their Old World kinsmen in both cultural practice and belief. They contained, in addition, a listing of Lehi’s fathers back through Joseph of old (see 1 Nephi 5:14), linking the Lehite colony genealogically with Abraham, Isaac, and Jacob, thereby perpetuating the patriarchal covenant—“the promises made to the fathers” (Doctrine and Covenants 2:2; Joseph Smith History 1:39)—in a new hemisphere. Finally, the plates contained a more extensive account of Old Testament peoples and events than the Bible (see 1 Nephi 5:12–13; 1 Nephi 13:23), although only a few precious remnants of this account are found in the Book of Mormon. 
- Among these remnants are teachings of four prophets not mentioned in the Bible: Zenos, Zenock, Neum, and Ezias. Other than the fact that they lived “since the days of Abraham” (Helaman 8:19), we know very little about these men. We are aware of their existence because their words or works are in many cases quoted or mentioned in passing by Nephite leaders.
Lehi and his family carried the plates with them all the way to the "promised land" in the Americas.
- King Benjamin [later] explained to his sons that “were it not for these plates [of brass], which contain these records and these commandments, we must have suffered in ignorance, even at this present time, not knowing the mysteries of God” (Mosiah 1:3).
When King Benjamin's father, Mosiah, discovered the people of Mulek, who had also fled Jerusalem to the Americas, their language had become corrupted, and they had nothing of the word of God.
Alma likewise explained to his son Helaman that the plates of brass “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” (Alma 37:8).
- ...the plates of brass contained writings of [Joseph who was sold into Egypt] not contained in the Bible. When Lehi in his old age spoke to and blessed his son Joseph, he recalled numerous covenants that the Lord had made with the Joseph..., including the promise “that out of the fruit of his loins the Lord God would raise up a righteous branch unto the house of Israel; not the Messiah, but a branch … to be remembered in the covenants of the Lord” (2 Nephi 3:5).
- Lehi also quoted many of Joseph’s prophecies about the record that Joseph’s seed would write, a great Joseph who would translate that record in the latter days, and also about Moses. The prophecies pertain to Lehi’s posterity up through the restoration of the gospel and the translation of the Book of Mormon. (See 2 Nephi 3:6–24.) Nephi added a postscript to these words, saying, “The prophecies which [Joseph] 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 Nephi 4:2).
Whereas the Old Testament prophecies of Christ are often veiled, the prophets of the plates of brass are bold in testifying of the coming of Jesus Christ and are specific as to his ministry. They provide plain evidence of the Savior’s words to his apostles: “All things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me” (Luke 24:44; see also Luke 24:25–27; Doctrine and Covenants 20:6–26). 
- Robert L. Millet, “The Plates of Brass: A Witness of Christ,” Ensign, Jan 1988, 26.