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 Alors la colère de l'Eternel s'enflamma contre Moïse, et il dit: N'y a-t-il pas ton frère Aaron, le Lévite? Je sais qu'il parlera facilement. Le voici lui-même, qui vient au-devant de toi; et, quand il te verra, il se réjouira dans son cœur.