This episode takes place when the forty years that Moses spent in the wilderness tending Jethro's sheep has come to an end, and God has sent him back to Egypt. However, Moses' faith is not really all that strong, so he complains to God that he does not know how to speak, which is interesting because, when Moses was in Egypt, he was a leader, a champion of men, apparently a general of the armies, and in line to become Pharaoh. Those responsibilities would entail that he communicate to others and maybe give speeches before thousands of people.
Yet now, suddenly, he does not know how to speak. Perhaps during that forty years, God had worked on him in such a way that, not that he had really forgotten how to speak, but he had learned enough about himself that he was no longer as self-confident as he had been in Egypt. Now he would have to put his confidence in somebody else. He may not have felt all that confident that God would be with him. So God came to his aid by providing his brother, Aaron, to do the speaking.
We all have failings of faith, but we should not feel overly bad about this because God supplies the need to overcome them in some way. He does not dump us as too weak but works things out another way. It is for experiences like this that the concept in I Corinthians 10:13 comes into play. God will make a way of escape that we may be able to bear, overcome, or endure our trials. In Moses' case, the way out was provided in the person of Aaron, who, apparently, had no fear of public speaking.