1 Now Moses was keeping the flock of his father-in-law, Jethro, the priest of Midian, and he led his flock to the west side of the wilderness and came to Horeb, the mountain of God.
2 And the angel of the LORD appeared to him in a flame of fire out of the midst of a bush. He looked, and behold, the bush was burning, yet it was not consumed.
This marks the turning point in Moses' life where God makes a huge gesture to get his attention. Living a presumably quiet life as a shepherd with a family, Moses is grabbed by God and conscripted to serve.
Moses' role is to be a prophet who speaks for God to Pharaoh and to Israel. Pharaoh must let Israel go and Israel must follow Moses as he leads them from captivity in Egypt to freedom in the promised land.
God promises to be with Moses and to guide him. Moses is very reluctant, not trusting in God to make things work. Instead, Moses is overly focuses on himself and his own inadequacies. As Moses walks with God, he learns to love and trust him more and more.