Thummim

Thummim - perfection (LXX., "truth;" Vulg., "veritas"), Ex. 28:30; Deut. 33:8; Judg. 1:1; 20:18; 1 Sam. 14:3,18; 23:9; 2 Sam. 21:1. 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. They were apparently material objects, quite distinct from the breastplate, but something added to it after all the stones had been set in it, something in addition to the breastplate and its jewels. They may have been, as some suppose, two small images, like the teraphim (comp. Judg. 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. They were probably lost at the destruction of the temple by Nebuchadnezzar. They were never seen after the return from captivity.

From Easton's Bible Dictionary (1897)