The breastplate held the Urim and the Thummim. These transliterated Hebrew words, mean “lights” and “perfections.” Together their names may mean “perfect knowledge.”
Exodus 28:30 states: “And you shall put in the breastplate of judgment the Urim and the Thummim, and they shall be over Aaron’s heart when he goes in before the LORD. So Aaron shall bear the judgment of the children of Israel over his heart before the LORD continually.” Here we see that the Urim and Thummim were placed in a breastplate which was worn by Aaron, the high priest, and that his breastplate was to reveal God’s judgment.
Leviticus 8:8 states: “Then he [Moses] put the breastplate on him [Aaron], and he [Moses] put the Urim and the Thummim in the breastplate.” Here again we see that the Urim and the Thummin were put in the breastplate.
And in Numbers 27:21 we read: “He [Joshua] shall stand before Eleazar the priest, who shall inquire before the LORD for him by the judgment of the Urim; at his word they shall go out, and at his word they shall come in, both he and all the children of Israel with him, all the congregation.” In this account the Urim is used to give judgment; to show what God wanted the Israelites to do.
The Urim and the Thummim were placed in the breastplate of judgment, which was worn by the high priest, and were the means by which God’s judgment or decision might be obtained in matters of national importance.
Besides these few facts little more is known. For some historical information on this subject see (Josephus, Book Three, Section Eight, Antiquities of the Jews).