Question: "What were the Urim and Thummim?"
Answer: The Urim ("lights") and Thummim ("perfections") were gemstones that were carried by the high priest of Israel on the ephod / priestly garments. They were used by the high priest to determine God's will in some situations. Some propose that God would cause the Urim and Thummim to light up in varying patterns to reveal His decision. Others propose that the Urim and Thummim were kept in a pouch and were engraved with symbols identifying yes / no and true / false.
It is unclear whether the Urim and Thummim were on, by, or in the high priest's ephod. No one knows the precise nature of the Urim and Thummim or exactly how they were used. The Bible simply does not give us enough information. References to the Urim and Thummim are rare in the Bible. They are first mentioned in the description of the breastplate of judgment (Exodus 28:30; Leviticus 8:8). When Joshua succeeded Moses as leader over Israel, he was to receive answers from God by means of the Urim through Eleazar the high priest (Numbers 27:21). The Urim and Thummim are next mentioned in Moses' dying blessing upon Levi (Deuteronomy 33:8). The following Scriptures likely also speak of the Urim and Thummim: Joshua 7:14-18; 1 Samuel 14:37-45; and 2 Samuel 21:1.