What does someone mean when they say "break a leg" in theater?

The term, of course, means to wish good luck. To “do well” or “have a great show” and is typically used before a stage performance, a show, or an audition. (I have never heard it used before filming a movie on any of the movies I’ve been involved with, but I guess it can be used in that sense too). But I’m sure you’re more interested in the origin of “break a leg”. Like many popular sayings and terms, the origin of “break a leg” is nebulous and disputed. The term “break a leg” was used originally, many say, to discourage evil spirits from deliberately causing one’s performance to suffer. According to this theory, wishing someone “good luck” would be invoking the “evil eye”. So “good luck” would actually cause bad luck for the actor. Thus, “break a leg”, by this logic, would be a wish for good luck.

Source: Today I Found Out