In Old Testament times, the three main channels of God's revelation were the sages, the priests, and the prophets. The priests had various duties, but their primary function until after the death of King Solomon was that of divining the will of God by the use of "Urim and Thummim."
Scholars think that these words refer to objects, perhaps stones, used by the high priest to ascertain the will of God in any important matter affecting the nation. One theory is that the stones were used to cast lots, with the manner of their falling somehow revealing the Lord's will.
Biblical scholar Horace R. Weaver gives this general description of the manner in which Urim and Thummim were used. The sacred container used for divining Urim and Thummim was a small box. "Yes-no" type questions were brought to the priest. He would place six stones, three white and three black, in the sacred container. The black stones might represent "no" and the white, "yes." The stones were mixed and three drawn at random. If all three were white then God's answer to the question was "yes"; three blacks implied a "no" answer; and a mix indicated that God gave no response that day.
For example, in 1 Samuel 14:37 Saul asks, " Shall I go down after the Philistines? Wilt thou deliver them into the hand of Israel?" The King James version of the Old Testament states, "He answered him not that day" : the stones, when drawn, were mixed in color. Other references to this sort of decision making are found in Exodus 28:90, where it states "and thou shalt put in the breastplate of judgment the Urim and the Thummim; and they shall be upon Aaron's heart, when he goeth in before the Lord: and Aaron shall bear the judgment of the children of Israel upon his heart before the Lord continually"; in Leviticus 8:8, we read, "and he put the breastplate upon him: also he put in the breastplate the Urim and Thummim."
This method of divining the will of God
became obsolete by 850 B.C.
Here are nine other verses that reference the Urim and Thummim:
Numbers 27:21
Moreover, he shall stand before Eleazar the priest, who shall inquire for him by the judgment of the Urim before the Lord. At his command they shall go out and at his command they shall come in, both he and the sons of Israel with him, even all the congregation.”
Deuteronomy 33:8
Of Levi he said, “Let Your Thummim and Your Urim belong to Your godly man, Whom You proved at Massah, With whom You contended at the waters of Meribah;
1 Samuel 23:9
Now David knew that Saul was plotting evil against him; so he said to Abiathar the priest, “Bring the ephod here.”
1 Samuel 28:6
When Saul inquired of the Lord, the Lord did not answer him, either by dreams or by Urim or by prophets.
1 Samuel 30:7
Then David said to Abiathar the priest, the son of Ahimelech, “Please bring me the ephod.” So Abiathar brought the ephod to David.
Ezra 2:63
The governor said to them that they should not eat from the most holy things until a priest stood up with Urim and Thummim.
Nehemiah 7:65
The governor said to them that they should not eat from the most holy things until a priest arose with Urim and Thummim.
Source: https://bible.knowing-jesus.com/topics/The-Urim-And-Thummim