This is from an article in the Church Magazine The Ensign published January 1986
The material constituting the eight chapters of the book of Moses is an extract from Joseph Smith’s translation of the Bible. More precisely, chapter one of Moses is an account of a revelation given to the Prophet Joseph just prior to, or at the commencement of, the translation, while Moses chapters two through eight constitute his translation of Genesis from chapter one through chapter six, verse thirteen. [Moses 1, 2–8; Gen. 1–6:13] The Joseph Smith translation of the Bible (hereafter identified as JST) began with Genesis and continued through the entire Bible to the book of Revelation. The initial draft of his Bible translation was made between June 1830 and July 1833. However, the short excerpt that we recognize as the book of Moses, being the early part of Genesis, was completed in its first draft by February 1831. 1
LDS.org "How We Got the Book of Moses"
BYU "History of the Book of Moses"
BYU-Idaho "The Book of Moses"
Joseph Smith's Translation of the Bible
A revision or translation of the King James Version of the Bible begun by the Prophet Joseph Smith in June 1830. He was divinely commissioned to make the translation and regarded it as “a branch of his calling” as a prophet.
Joseph Smith often used the words "translated" and "translation," not in the narrow sense alone of rendering a text from one language into another, but in the wider senses of "transmission," having reference to copying, editing, adding to, taking from, rephrasing, and interpreting. This is substantially beyond the usual meaning of "translation." When he said the Bible was not translated correctly, he not only was referring to the difficulties of rendering the Bible into another language but he was also observing that the manuscripts containing the text of the Bible have suffered at the hands of editors, copyists, and revisionists through centuries of transmission. Thus, the available texts of the Bible are neither as complete nor as accurate as when first written.
Although not the official Bible of The Church of Jesus Christ of Latter-day Saints, the JST offers many interesting insights and is an invaluable aid to biblical interpretation and understanding. It is a most fruitful source of useful information for the student of the scriptures. It is likewise a witness for the divine calling and ministry of the Prophet Joseph Smith.
Bible Dictionary "Joseph Smith Translation"
BYU "Joseph Smith Translation of the Bible (JST)