What does 'break a leg' mean?

Break a leg is theatrical slang for good luck. When someone told you to break a leg, it means that person wants to send you a good fortune especially when you’re about to perform some task like performance or exam.

The phrase 'break a leg' is a jovial encouragement, said to actors for good luck before they go on stage, especially on an opening night. The term 'break a leg' appears to come from the belief that one ought not to utter the words 'good luck' to an actor.