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 be thy spokesman - Literally, He shall speak for thee (or in thy stead) to the people.
He shall be to thee instead of a mouth - He shall convey every message to the people; and thou shalt be to him instead of God - thou shalt deliver to him what I communicate to thee.