Boyd’s Bible Dictionary:
(light and perfection). From the way these mysterious words are spoken of (Ex. 28:30), and (Lev. 8:8), compared with (Ex. 28:15-21), they appear to denote some material things, separate from the high priest’s breastplate and its gems, and previously well known. Their purpose seems to be indicated (Num. 27:21; 1 Sam. 28:6), and, since they were connected with the ephod (1 Sam. 22:14-15; 23:9-12; 30:7-8), it may be inferred they were consulted to ascertain the will of Jehovah, and that they were preserved in the bag of the high priest’s breastplate to be borne “upon his heart before the Lord continually” (Ex. 28:30). Not in use after the captivity (Ezra 2:63; Neh. 7:65; Hos. 3:4).
Concise Bible Dictionary:
The signification of these Hebrew words is “lights” and “perfections.” They were distinct from the gems on the breastplate, for Moses put the breastplate upon Aaron, “also he put in [or ‘on’] the breastplate the Urim and the Thummim” (Lev. 8:8). It is clear that God answered questions by means of the Urim and Thummim (Num. 27:21; Deut. 33:8; 1 Sam. 28:6). On the return of the Jews from Babylon some, who claimed to be priests but could not show their genealogy, were not allowed to eat of the holy things until there should stand up a priest with Urim and Thummim, and an answer be obtained from God. This great privilege has never yet been restored (Ezra 2:63; Neh. 7:65).
It may be remarked that there is no record as to the construction of the Urim and Thummim, nor of their form. The first mention of them is in Exodus 28:30: “Thou shalt put in [or ‘on’] the breastplate of judgment the Urim and the Thummim; and they shall be upon Aaron's heart when he goeth in before the Lord,” as if God had given them to Moses, and had merely to tell him what to do with them—if indeed they were material things; but what they were, and how the answers were given, is not revealed. When Israel is restored, Christ Himself will take the place of the ancient Urim and Thummim.