A cue is the long stick used for playing pool. A cue is also a reminder or stimulus to do something.
In pool, you use the cue to hit the white ball into the red ball, aiming for a pocket. Other cues make things happen too. In acting, a lighting change or another actor's line could be a cue for you to enter the stage. During rehearsal, the actors need to learn their cues to make sure their timing is right. A cue can also be a clue or reminder of something. Anytime someone has good timing, they're on cue.