When Moroni was finishing the Book of Mormon record, he was commanded to seal up some of the plates, and Joseph Smith was later commanded not to translate them. This sealed portion contains the complete record of the vision of the brother of Jared (see Ether 4:4–5). This vision included “all things from the foundation of the world unto the end thereof” (2 Nephi 27:10–11; see also Ether 3:25). So basically the Lord revealed to the brother of Jared the history of mankind, and the sealed portion of the plates was Moroni’s translated copy of it.
Few people have seen the sealed record—for instance, the Nephites in the land Bountiful at the Savior’s coming (see Ether 4:1–2) and Moroni (see Ether 12:24). The Lord said the sealed portion would be revealed to the world “in mine own due time” (Ether 3:27). He also said it would “not go forth unto the Gentiles until the day that they shall repent of their iniquity, and become clean before the Lord” (Ether 4:6; see also 2 Nephi 27:8).
According to Joseph Smith’s associates who saw the golden plates, anywhere from half to two-thirds of all the plates were in the sealed portion (see Kirk B. Henrichsen, “What Did the Golden Plates Look Like?” New Era, July 2007, 31).