Seer stones, also called Urim and Thummim in the Mormon Church, are “instruments prepared by God to assist man in obtaining revelation and in translating languages. In the Hebrew language, the words [Urim and Thummim] mean ‘lights and perfections.’ The Urim and Thummim consist of two stones set in silver bows and are sometimes used with a breastplate (Doctrine and Covenants 17: 1; JS-H 1: 35, 42, 52).” (LDS Guide to the Scriptures).A seer is a prophet authorized to use these special interpreters. The Urim and Thummim and a breastplate are mentioned occasionally in the Old Testament, but it seems apparent that they were either lost or taken from the Jews before the time of Jesus Christ. Some of the Nephite prophets in the Book of Mormon also are mentioned as having seer stones.
Joseph Smith used the Urim and Thumim as he translated the Book of Mormon. He briefly described them in Joseph Smith History 1:35: “there were two stones in silver bows-and these stones, fastened to a breastplate, constituted what is called the Urim and Thummim … the possession and use of these stones were what constituted ’seers’ in ancient or former times … God had prepared them for the purpose of translating the book.” Joseph Smith does not give any accounts of the exact process used to translate with the Urim and Thummim, but instead stresses the fact that his ability to translate was a gift from God, and worked through the Spirit.
Joseph Smith also later commented that seer stones are not essential to translation or revelation. Rather, they were provided for him when he was inexperienced in receiving revelation. As Joseph Smith continued to translate and became better acquainted with revelation, he no longer needed to use them.