Ex 4:10 And Moses said to the Lord, O Lord, I am not a man of words; I have never been so, and am not now, even after what you have said to your servant: for talking is hard for me, and I am slow of tongue.

 11 And the Lord said to him, Who has made man's mouth? who takes away a man's voice or hearing, or makes him seeing or blind? Is it not I, the Lord?

 12 So go now, and I will be with your mouth, teaching you what to say.

 13 And he said, O Lord, send, if you will, by the hand of anyone whom it seems good to you to send.

 14 And the Lord was angry with Moses, and said, Is there not Aaron, your brother, the Levite? To my knowledge he is good at talking. And now he is coming out to you: and when he sees you he will be glad in his heart.

 15 Let him give ear to your voice, and you will put my words in his mouth; and I will be with your mouth and with his, teaching you what you have to do.

 16 And he will do the talking for you to the people: he will be to you as a mouth and you will be to him as God.

 17 And take in your hand this rod with which you will do the signs.

Back in Exodus 2:14, when Moses intervened in the fight between two Israelites, he was asked: "Who made you a prince and a judge over us?"

Here, Moses wrote this incident to show the Israelites that he did not arrogate to himself to become Israel's leader and human deliverer. The whole thing was God's idea and call. In fact, he shows here that, at first, he was hesitant to obey the call. He did not readily obey and go. The Israelites should never think that he made himself their leader and human deliverer.

The arrangement between God, Moses and Aaron will be something like this: God will speak to Moses; Moses will speak to Aaron; Aaron will speak to the people.

Ex 4:1 Then Moses answered and said, "But suppose they will not believe me or listen to my voice; suppose they say, 'The LORD has not appeared to you.'"

 2 So the LORD said to him, "What is that in your hand?" He said, "A rod."

 3 And He said, "Cast it on the ground." So he cast it on the ground, and it became a serpent; and Moses fled from it.

 4 Then the LORD said to Moses, "Reach out your hand and take it by the tail" (and he reached out his hand and caught it, and it became a rod in his hand),

 5 "that they may believe that the LORD God of their fathers, the God of Abraham, the God of Isaac, and the God of Jacob, has appeared to you."

 6 Furthermore the LORD said to him, "Now put your hand in your bosom." And he put his hand in his bosom, and when he took it out, behold, his hand was leprous, like snow.

 7 And He said, "Put your hand in your bosom again." So he put his hand in his bosom again, and drew it out of his bosom, and behold, it was restored like his other flesh.

 8 "Then it will be, if they do not believe you, nor heed the message of the first sign, that they may believe the message of the latter sign.

 9 "And it shall be, if they do not believe even these two signs, or listen to your voice, that you shall take water from the river and pour it on the dry land. And the water which you take from the river will become blood on the dry land."

Moses was concerned about his credentials. And so, in order to establish it before the eyes of the Israelites, God granted him three signs. Three, in order to establish and demonstrate it without a doubt.

The first sign was of a serpent. His rod turned into a serpent and yet it turned back into his rod again when he took it by the tail.

Serpents represented the power of Egypt. The Pharaohs of Egypt would wear a metal cobra around their heads symbolizing the nation and its power. Moreover, it was worshipped in Egypt.

The ability of Moses to turn the serpent into his useful rod would encourage confidence among the Israelites in Moses. It would truly demonstrate that God was with him. It would send a message to the Israelites that this Moses can handle the power of Egypt.

The second sign was that of leprosy suddenly infecting his hand and yet was immediately healed back to health again.

Probably, this was Hansen's disease and it was incurable during their time. The gods of the Egyptians were helpless against leprosy, and, yet, here is Moses, with the ability to deal with the disease just like that. It would demonstrate that God was with him and that his God was powerful over the gods of Egypt.

The third sign was that of water from the Nile turning into blood.

The Nile was the source of life for Egypt. The entire nation was dependent on it for its survival. The Egyptians attributed the benefits of the Nile with their god Osiris.

Moses' ability to turn the water of the Nile into blood was a sign that the life source of Egypt was in his hands. He got Egypt by the throat, so to speak. And this would demonstrate the power of God  over their god Osiris and that this God was with Moses.