Read Exodus 4:16 Using Other Translations
And he shall be thy spokesman unto the people: and he shall be, even he shall be to thee instead of a mouth, and thou shalt be to him instead of God.
He shall speak for you to the people, and he shall be your mouth, and you shall be as God to him.
Aaron will be your spokesman to the people. He will be your mouthpiece, and you will stand in the place of God for him, telling him what to say.