Aaron and his successors as high priest were given control over the Urim and Thummim by which the will of God could be determined. God commissioned the Aaronide priests to distinguish the holy from the common and the clean from the unclean, and to teach the divine laws (the Torah) to the Israelites.
The passage describes how Aaron and his descendants held authority over the Urim and Thummim, which was used to understand God's will. They were also responsible for separating sacred from profane and instructing the Israelites in the divine laws (the Torah).