What's something that would really make you sit up and pay attention? Something really unexpected, right? The element of surprise is, well, surprisingly powerful. If you suddenly saw monkeys swinging in the trees outside your classroom window, it'd be pretty hard to pay attention to the teacher! (At least, if you don't live in a rain forest or other places where monkeys live.)
God used the power of surprise in the Bible too. He wanted to show Moses that He is powerful, and that He has the strength to do what He knows is right. So how did He get Moses' attention? The burning bush!
Now, usually when something is being burned, it turns to ash and is destroyed, right? The surprising bit about the burning bush is that, well, it wasn't being destroyed. That's a pretty big deal, it's probably even more surprising than seeing monkeys at school! It definitely made Moses pay attention, which is exactly what God wanted, and that's when He gave Moses the mission of freeing the Israelites from slavery in Egypt. The miracle proved to Moses that God had great power, that He was more than powerful enough to help Moses accomplish the mission.
Moses knew that he couldn't do God's mission on his own; he said himself "I am not a great man! How can I be the one to go to Pharaoh and lead the Israelites out of Egypt?” (Exodus 3:11) Moses tried to give all kinds of excuses why he couldn't do it, like "they won't believe me", or "how will they know You sent me?" Eventually he just straight up begged God not to send him, but the Lord wasn't going to hear any of it. He gave answer for each of Moses' excuses, and Moses finally obeyed God.
You can read more about the burning bush and Moses' mission here in Exodus 3 and Exodus 4.