What did aristotle mean by the first cause?

Based on my research, Aristotle believes that his "Prime Mover" is God. He believed that everything that has happened, every "cause", had to have a cause before it. Something had to cause the first cause. However, the first cause can not be in the same formula or be part of the same equation. The rules of the first cause can not applied to the "causer" of the first cause. God has no beginning or end so time doesn't apply to God.