Here is a little more of the context:
Exodus 3
7 And the LORD said, I have surely seen the affliction of my people which are in Egypt, and have heard their cry by reason of their taskmasters; for I know their sorrows; 8 And I am come down to deliver them out of the hand of the Egyptians, and to bring them up out of that land unto a good land and a large, unto a land flowing with milk and honey; unto the place of the Canaanites, and the Hittites, and the Amorites, and the Perizzites, and the Hivites, and the Jebusites. 9 Now therefore, behold, the cry of the children of Israel is come unto me: and I have also seen the oppression wherewith the Egyptians oppress them. 10 Come now therefore, and
I will send thee unto Pharaoh, that thou mayest bring forth my people the children of Israel out of Egypt. . .
Exodus 4
10
And Moses said unto the LORD, O my Lord, I am not eloquent, neither heretofore, nor since thou hast spoken unto thy servant: but I am slow of speech, and of a slow tongue. 11 And the LORD said unto him, Who hath made man's mouth? or who maketh the dumb, or deaf, or the seeing, or the blind? have not I the LORD? 12 Now therefore go, and I will be with thy mouth, and teach thee what thou shalt say. 13 And he said, O my Lord, send, I pray thee, by the hand of him whom thou wilt send. 14 And the anger of the LORD was kindled against Moses, and he said, Is not Aaron the Levite thy brother? I know that he can speak well. And also, behold, he cometh forth to meet thee: and when he seeth thee, he will be glad in his heart. 15 And thou shalt speak unto him, and put words in his mouth: and I will be with thy mouth, and with his mouth, and will teach you what ye shall do. 16 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. 17 And thou shalt take this rod in thine hand, wherewith thou shalt do signs.
God commanded Moses to go speak boldly to Pharaoh. Moses didn't want to go because he felt that he was not a clear bold speaker. God told Moses that He (God) created man's mouth and voice, and would provide Moses with the necessary words to speak. Still, Moses refused to obey God. God again ordered Moses. God told Moses that He (God) would provide Moses with someone to help him speak to Pharaoh. That someone was Aaron, Moses' brother. Aaron was a very skilled speaker, and he was willing to help Moses. Together, Moses and Aaron would be a strong team to face Pharoah.
I hope this helps.