What does "Break a leg" mean?
Correct answer: Good luck

Player #33161631
Rey, it is considered unlucky to wish someone 'good luck' just before they go on stage as it might jinx the performance. So jokingly wishing them bad luck, as in 'break a leg' is intended to work the superstition backwards and bring them good luck after all.
In the UK it is often said before someone undertakes a specific task or 'role', as if they are going on stage themselves.

K9Queen
It's considered "bad luck" to actually say "good luck" to an actor before they go on stage.

Queen Esther
Player #33161631, Yeah! that's how it is not just in the UK but in the USA! too! it's like a saying of good luck! they're not actually saying to break a leg but it means that the person hopes u do a good job! on your performance!

BallonX107
And here I was thinking that when someone says 'break a leg' it means to hurry up!

Ron
Rey, Break a leg referred to the old practice of stomping your foot rather than clapping. If you really liked something and stomped hard enough, you could "break your leg"

Marcy
Same saying as give your best