An action, or "Cause of Action", is a claim that can be asserted in a court. A plaintiff has a cause of action if she has a claim that can be the proper subject of a law suit.

"Cause of action" confused me to no end my first year of law school. What the was "causing" the action? And where was the "action?"

My point is that you cannot think of "cause of action" as consisting of separate words - it is a single word in itself. "Cause of action" is a legal magic word. There is no "cause." And there is no "action." There is only a claim that can be asserted in court.