A "curve ball" is a kind of pitch in baseball. When you throw a curve ball, the ball doesn't travel in a straight line; it curves downward as it gets close to the batter. That makes the ball hard to hit.

There's also a metaphorical meaning of "curve ball". People use it to talk about anything that was unexpected:

Don't be surprised if they throw you a curve ball in the interview; they just want to see how well you respond under pressure.

In this example, a "curve ball" means an unexpected question.