As part of the description of the breastplate, Ex. 28:30 reads as follows in the NIV: "Also put Urim and Thummim in the breastpiece, so that they may be over Aaron's heart whenever he enters the presence of the LORD. Thus Aaron will always bear the means of making decisions for the Israelites over his heart before the LORD." (In the KJV: "the Urim and the Thummim" -- my emphasis)
What are these things, Urim and Thummim? There is no further description of them in this chapter. In Number 27, Joshua inquires of the Urim before the Lord when making decisions. In 1 Sam. 28, Saul tries to receive an answer from God via dreams, prophets, or the Urim. They appear a few other times in scripture (listed in Biblegateway.com). But we actually know almost nothing about them. Given the emphasis on the other aspects of Aaron's clothing, this mystery is all the more baffling.