Essentially means to stay attentive and focused on a task. This can be used in various ways. One example would be: "He is really on the ball since he started eating salad every day." Another common way of using "<strong>On the ball</strong>" is to suggest that a person should "Stay <strong>on the ball</strong>."
This can also be used in a conversation referring to oneself. “I really have to stay on the ball to handle all of my tasks.” The phrase is likely to have originated from sports. A person playing a game with a ball should generally stay focused “on the ball.”