Aaron must do the talking for Moses, as his prophet (Exodus 7:1). Aaron speaks well (Exodus 4:14). About the relationship between Moses and Aaron’s speaking, see Exodus 4:14–16.
Aaron must pass on God’s command, that Pharaoh must release the Israelites (Exodus 6:11).
2 You shall speak all that I command you, and your brother Aaron shall tell Pharaoh to let the people of Israel go out of his land.