"Break a leg" is an expression from Britain. (Here, an "expression" means a particular way of saying something.)
It can mean about the same as "good luck".
This expression was used in theaters by actors. Before an actor went on stage, other actors could wish that he or she had good luck in their work. Then, a co-worker could say "Break a leg!".
When the expression is used like this, it is a form of irony.