Now go and I will tell you what to say and Help you speak well.
God had chosen Moses, but Moses was a man of little courage. He came up with all kinds of excuses as to why he couldn't do what God wanted him to. The most famous, or the most prominent was his inability to speak well. Some say Moses had a speech impediment, some say he must have stuttered (which I guess is a speech impediment), others say Moses was just being modest; as Paul was when he said he did not speak with eloquence.
God reminds Moses that it was he who made his mouth. Thus God knew what it was capable of, and what it's limitations were. It is often through our weaknesses that God works. This way we can not take the credit for what we have done for we know that it was God's power that gave us the ability to do that thing which we believed we were incapable of.
God often calls us where we do not think we can go. The good news is that we are not going there alone. There is a famous saying "what doesn't kill you makes you stronger." When we follow God where we didn't think we can go we find out what we are capable of when God is at our side.