There’s a long running saying in the theater culture, where instead of wishing someone good luck, you are supposed to say, “Break a leg.” It’s actually considered bad luck to literally wish someone good luck. According to The Phrase Finder, it could potentially imply that this person has put on such a good performance that they literally bend their knees “in a bow or to curtsey to acknowledge applause.” Another meaning could be that the actors may get their big “break” from this performance and their performance leads them to success.
I think that this superstition reflects the mental mindset of people who are in performance or show business. There’s never a guarantee of success, and so much of the career in acting or performance is based on luck, chance, and the hope that you will be discovered. Therefore, it’s important to be careful and superstitious, so that the performer’s mind is filled with positive energy, as so much of their career is dependent on their optimism and mental drive.