We could generally describe the role of God's priests as follows:
1. These were ordained by God to offer sacrifices–
"For every high priest taken from among men (all the Jewish priests were taken from the Kohan family of the tribe of Levi) is appointed on behalf of men (set apart for their work to represent their people before God) in things pertaining to God, in order to offer both gifts and sacrifices for sins (priests were able to sympathize with the people, because they were subject to the same weaknesses, and also had need of the forgiveness of their own sins)," Hebrew 5:1 (NASB).
2. Priests were to teach the Law–
"And that ye may teach the children of Israel all the statutes which the LORD hath spoken unto them by the hand of Moses," Leviticus 10:11.
"… for the law shall not perish from the priest, nor counsel from the wise, nor the word from the prophet," Jeremiah 18:18.
3. Priests determined whether a person was sick or healthy–
"When a man shall have in the skin of his flesh a rising, a scab, or bright spot, and it be in the skin of his flesh like the plague of leprosy; then he shall be brought unto Aaron the priest, or unto one of his sons the priests: And the priest shall look on the plague in the skin of the flesh… and the priest shall look on him, and pronounce him unclean." Leviticus 13:2,3
Deuteronomy 33:10 summarizes priestly duties: "They shall teach Jacob thy judgments, and Israel thy law: they shall put incense before thee, and whole burnt sacrifice upon thine altar."