Imagine going up to the biggest, meanest kid you know, and telling them that what they’re doing is wrong. That would be pretty scary, wouldn't it? It's exactly what Moses did when he told Pharaoh to let the Israelites (the people God chose for Himself) leave Egypt. Pharaoh didn't want them to leave because they were his slaves and did a lot of work nobody wanted to do. Since he ruled the most powerful country around at that time, it was a big deal for Moses to tell him what to do. So why’d he do it? Wasn’t he scared?
Moses was scared. He made a lot of excuses why he couldn't do it, and eventually God got angry at hearing all of his excuses! Moses did obey God, and because of his obedience, God also gave him the ability to set the Israelites free. Moses did some pretty incredible stuff for God, not just in Egypt, but after that too, when all of the Israelites were wandering in the desert on their (very long) trip back to Israel, the land God promised to give them.
Read more about Moses in Exodus chapter 4 in your Bible!