Be alert to improve one’s performance. Pay attention. Also used as ‘get on the stick’. ‘Ball’ is used to mean paying attention, being alert. May have been a baseball reference where a batter must watch the ball closely to make a good swing. The reference to ‘stick’ may just as well mean a bat (stick). (Compare to On one’s toes.)

“You’ve worked here for 3 months and still can’t perform your job, so I want you to get on the ball or you are out of here.”