It's a theatrical term from a long time ago. It refers to an actor's reaction to the crowd's applause for him for his performance at the end of the play. In acknowledging the audience's appreciation he would bow and genuflect to them; bending or "breaking" a leg. Hence before a performance an actor would be told "hope you (have to) break a leg", i.e. do such a good job that the audience's reaction would be such that it required him bowing at the end of the play.