Moses is the prophet. He receives the messages from God (see Exodus 3:18–20). But Aaron gets the message from Moses; Moses is to Aaron what God is to him. Aaron is the prophet of Moses, like Moses is the prophet of God (Exodus 7:1–2). He is the mouth of Moses, like Moses is the mouth of God.
Aaron has to speak on behalf of God. That is why it is important to tell God’s words, not his own not only to Israel, but also to Pharaoh (see Exodus 7:1–2).
14 Then the anger of the LORD was kindled against Moses and he said, “Is there not Aaron, your brother, the Levite? I know that he can speak well. Behold, he is coming out to meet you, and when he sees you, he will be glad in his heart.