When actors perform, they want to do their best. People say 'break a leg' because it's a special way of wishing them good luck, like magic! It doesn't mean they should actually hurt themselves.
Why 'Break a Leg'?
Long ago, people thought that if you wished someone bad luck before a show, they would do better. So instead of saying 'good luck,' they said 'break a leg', the opposite of good luck. Now it's just a fun tradition!