It means you are alert and paying attention. You are ready to do something. A manager might say to their employees "It is going to be very busy today, I need you to be on the ball."

"On the ball" means that someone is good at solving problems, organizing events, or being generally useful. Imagine a person balancing on a ball; they would have to be very nimble and skilled, reacting quickly to every new movement of the ball. A person who is "on the ball" also has to be able to react to new developments and problems in order to solve them.