Idiom Definition:
"to be on the ball"
to be quick to understand
to be quick to react to situations usually due to being prepared
Synonyms:
active, alert, alive, attentive, awake, bright, competent, knowledgeable, intelligent, observant, perceptive, quick, ready, sharp, vigilant, vital, watchful, wise
This baseball player is quick to assess the situation and reacts quickly by running to catch the ball.
He
is literally
on the ball.