Exodus 3:13
And Moses said unto God, Behold, when I come unto the children of Israel, and shall say unto them, The God of your fathers hath sent me unto you; and they shall say to me, What is his name? what shall I say unto them? And Moses said unto God, Behold, when I come unto the children of Israel, and shall say unto them, The God of your fathers hath sent me unto you; and they shall say to me, What is his name? what shall I say unto them?
Moses asks this question at Horeb, the mountain of God, while keeping the flocks of Jethro in retirement where God was training him as a spiritual shepherd of Israel. He asks after God appears in the burning bush and commissions him to go to Pharaoh to bring Israel out of Egypt to a good land flowing with milk and honey. He asks after God promises to be with him, with the token that Israel shall serve God upon that mountain.
The question comes when the time has fully come for Moses to exchange the shepherd's staff for the rod of God to be made powerful in signs and wonders for delivering Israel from oppression and preserving them when pursued by enemies. Before this, Moses had tried to do the work by force and might, killing an Egyptian who was fighting with an Israelite and hiding him in the sand and expecting Israel to recognize him as their deliverer, but he was not ready. God then sent him away for forty years to act as a keeper of sheep in the rocks and mountains and desert, hunting with patience for stray sheep, until he learned to be a tender, patient, loving, faithful shepherd.
Moses shrank from standing before Pharaoh in God's name to demand Israel's release because he expected deliverance by warfare with himself as general at the head of Israel, not by God's own mighty power magnified before Israel and the Egyptians to teach that there is a living God with power to save and to destroy. The Pharaoh he must face is not the one who decreed his death, for that king is dead and another reigns. In asking what to say when the children of Israel ask for his name, Moses anticipates that the Israelites, many of whom knew little of God, will ask which God sent him because they are marked by blindness and unbelief.
God answers by revealing himself to Moses as Jehovah, as I AM THAT I AM, telling him to say that I AM sent him, and then commands him to go and deliver the children of Israel because now, being prepared to lead Israel, he has the education needed for the special work.