And he shall be thy spokesman unto the people
And open to them Moses's commission from God, and the end of his mission into Egypt, and to them, and declare what signs had been, and would be done, in confirmation of it: and he shall be, [even] he shall be to thee instead of a mouth;
or an interpreter, as all the Targums explain it, and so Jarchi; as he was an orator and master of language, he should speak to the people for Moses, and explain his sense and meaning, and put it into plain, proper, easy language, to be understood by the people; and this may be done where a different language is not spoken, but the same in plainer words, in more pertinent expressions, and better pronounced, and this is repeated for the certainty of it: and thou shall be to him instead of God;
Aaron was to stand between Moses and the people, and speak for him; and Moses was to stand between God and Aaron, and in God's stead, and tell him what orders he had received from him, and which he should communicate; and so some Jewish writers F1 interpret it of his being to him instead of a master or teacher, one that received doctrine from the Lord, and instructed him in it, and taught him the mind and will of God: or, as Onkelos paraphrases it; "for a prince", and so Jarchi, a civil magistrate, one that had the power of life and death; the administration of civil affairs belonged to Moses, and Aaron, though the elder brother, was subject to him; and in this sense Moses was a god to him; and so in after times, the judges of Israel, they that sat in Moses's chair, were called gods, ( Psalms 82:1 ) .
F1 Targum Jon. Jerus. & Abendana in loc.