The Urim and Thummim were instruments used in ancient Israel to determine the will of the Lord. The meanings of these words in Hebrew are a bit obscure but seem to be something like “lights” (for Urim) and “perfections” (for Thummim). In the Old Testament, these instruments were closely associated with the Aaronic Priesthood and the temple.
Exodus 28 describes the special liturgical clothing Aaronic high priests wore and the Urim and Thummim were part of this special clothing: they were placed in the breastplate and worn on the chest (Exodus 28:30). The book of Exodus describes this as bearing the “judgment of the children of Israel upon his [the priest’s] heart before the Lord continually.” In 1 Samuel 28:6, Urim are listed as one of the ways that King Saul expected to receive revelation but was unable to do so.
Biblical scholarship has suggested that the Urim and Thummim were perhaps some kind of sacred lot cast to obtain divine will (see 1 Nephi 3:11 and Acts 1:23–26). Latter-day Saint revelation connects the Urim and Thummim with receiving revelation through seer stones (see Joseph Smith—History 1:35). This connection clarifies the use of the Urim and Thummim to receive a vision in Abraham 3:1–2.