priest is supposed to be a spokesman for God. A priest is supposed to have the ability & right to communicate with God for others. Of course, we know that during the days of Moses, God Himself ordained a family of priests in Israel from the tribe of Levi. Aaron (Moses' brother) became the father of a long line of sacred ministers who stood between God & the people. The Jews to whom this book is addressed would have been very familiar with the role of the Levites in worship & sacrifice. And, the Jews properly reverenced these men very highly for the sake of their office. But here we find that the priesthood of Aaron's family is neither the only, nor is it the highest order of priestly privilege.
There were indeed priests prior to Aaron. In fact, it seems that the eldest son in every godly pre-Aaronic family was something of a priest within his own house. It is worth noting that Abel, Noah, Abraham, Isaac & Jacob all made sacrifices to God & communicated with Him in ways that matched the fulfilling of priestly duties. Moses' father-in-law (Reuel) was a priest too (Exodus 2:16). As we see here, there was a mysterious priest who lived in Old Testament times to whom Abraham paid tithes. His name was Melchizedek. And indeed, Christ Himself is the greatest of all priests. His eternal priesthood is far superior to the priesthood of every previous priest.