In the passage you're referring to, Exodus 7:1, when the Lord says to Moses, "See, I make you as God to Pharaoh, and Aaron your brother shall be your prophet," Aaron is assigned the role of a prophet, not a priest. The term "prophet" here indicates that Aaron would speak on behalf of Moses to Pharaoh. Aaron later serves as a high priest in Israel, but his initial role is that of a prophet.