From the Wikipedia article on Joseph Smith’s seer stone:
According to Latter Day Saint theology, seer stones were believed to have been used by Joseph Smith, as well as ancient prophets, to receive revelations from God. Members of The Church of Jesus Christ of Latter-day Saints believe that Joseph Smith used a seer stone in the Book of Mormon translation effort.
Seer stones are mentioned in the Book of Mormon in the Book of Mosiah, where they are also called “interpreters” and described as being used by seers to translate and receive revelations. The term “Urim and Thummim” is usually used by Latter Day Saints members to refer to the “interpreters” mentioned in the Book of Mormon. Some Latter Day Saints use the term Urim and Thummim and seer stones interchangeably.
David Whitmer said, regarding Smith’s translation of the Book of Mormon:
Joseph Smith put the seer stone into a hat, and put his face in the hat, drawing it closely around his face to exclude the light; and in the darkness the spiritual light would shine. A piece of something resembling parchment would appear, and on that appeared the writing. One character at a time would appear, and under it was the interpretation in English. Brother Joseph would read off the English to Oliver Cowdery, who was his principal scribe, and when it was written down and repeated to Brother Joseph to see if it was correct, then it would disappear, and another character with the interpretation would appear. Thus the Book of Mormon was translated by the gift and power of God, and not by any power of man.
The Actual Seer Stone
In 2015, the Church of Jesus Christ of Latter-day Saints published pictures of the seer stone that Joseph Smith used to translate the Book of Mormon.