Ex 3:1 Now Moses was tending the flock of Jethro his father-in-law, the priest of Midian. And he led the flock to the back of the desert, and came to Horeb, the mountain of God.

 2 And the Angel of the LORD appeared to him in a flame of fire from the midst of a bush. So he looked, and behold, the bush was burning with fire, but the bush was not consumed.

 3 Then Moses said, "I will now turn aside and see this great sight, why the bush does not burn."

 4 So when the LORD saw that he turned aside to look, God called to him from the midst of the bush and said, "Moses, Moses!" And he said, "Here I am."

The bush represents Israel. As a bush is weak, so Israel was weak compared to the mighty tree of Egypt. That the bush is burning represents Israel's fiery affliction in Egypt. God described Israel's oppression in Egypt as being in a fiery furnace.

De 4:20 "But the LORD has taken you and brought you out of the iron furnace, out of Egypt, to be His people, an inheritance, as you are this day.

It was a common sight in the Sinai desert for bushes to spontaneously burst into flames but this one was unusual. It was not consumed. That the bush was burning and yet it was not consumed is an apt picture of Israel in Egypt; Egypt oppressed Israel yet Israel was never brought to annihilation. The secret of their preservation? God was in their midst. Just as God spoke from the midst of the bush so was God in the midst of Israel miraculously preserving her.

That God spoke from the midst of the bush, calling Moses, so God calls Moses to go back to where Israel was. Moses was to go back to Egypt. And just as Moses tended the flock of Jethro so Moses will now lead Israel out of Egypt as a shepherd.