When Nephi and his family traveled from Jerusalem (the “Old World”) to the promised land (the “New World”), they brought scriptures. They had the commandments and the teachings of many prophets. These were called the brass plates. In the Old World, these scriptures became part of the Bible.
In the promised land, Nephi wrote down what his father taught, what happened to his family, what he learned from Heavenly Father, and what he learned from the brass plates. The things Nephi wrote became part of the Book of Mormon.