What is a…
Hebrew: תֻּמִּים or תוּמִים —transliteration: tummim or thummim
This object, an emblem of complete Truth, was placed in the high priest’s breastplate along with the Urim (meaning: light or lights) and placed over the ephod. The high priest served as the mediator between God and man.
Scripture does not clearly reveal its appearance or how it functioned, but it seems to clearly have been used by God in some way to communicate His will to the high priest, possibly in the form of a simple yes or no answer, or of innocent or guilty.
What the “Urim and Thummim” were cannot be determined with any certainty. All we certainly know is that they were a certain divinely-given means by which God imparted, through the high priest, direction and counsel to Israel when these were needed. The method by which this was done can be only a matter of mere conjecture.
Some suppose that they were 2 small images, like the teraphim (compare Judges 17:5; 18:14, 17, 20; Hos. 3:4), which were kept in the bag of the breastplate, by which, in some unknown way, the high priest could give forth his divinely imparted decision when consulted.
“You shall put in the breastpiece of judgment the Urim and the Thummim, and they shall be over Aaron’s heart when he goes in before the Lord; and Aaron shall carry the judgment of the sons of Israel over his heart before the Lord continually.” —Exodus 28:30 NASB
“Then Moses had Aaron and his sons come near and washed them with water. He put the tunic on him and girded him with the sash, and clothed him with the robe and put the ephod on him; and he girded him with the artistic band of the ephod, with which he tied it to him. He then placed the breastpiece on him, and in the breastpiece he put the Urim and the Thummim.” —Leviticus 8:6-8 NASB
“You shall put some of your authority on him, in order that all the congregation of the sons of Israel may obey him. 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.” —Numbers 27:20-21 NASB
“When Saul saw the camp of the Philistines, he was afraid and his heart trembled greatly. When Saul inquired of the Lord, the Lord did not answer him, either by dreams or by Urim or by prophets.” —1 Samuel 28:5-6 NASB
“These searched among their ancestral registration, but they could not be located; therefore they were considered unclean and excluded from the priesthood. The governor said to them that they should not eat from the most holy things until a priest stood up with Urim and Thummim.” —Ezra 2:62-63 NASB (compare Nehemiah 7:65 NASB)