Some of the pieces of the priestly garments were not only beautiful but also significant. For example, one part of the high priest’s uniform was the breastpiece. On the front of the breastpiece were attached 12 precious stones, each inscribed with the name of a tribe of Israel. This symbolized how the high priest represented all the people before God. The breastpiece also contained pockets that held two stones or plates called the Urim and the Thummim, which were consulted to determine God’s will for the nation.
–NIV Life Application Study Bible