What does brake a leg mean and how does it affect you do they really mean it or are they saying do a great job or something like that?

Theatrical superstition is that wishing a person "good luck" is considered bad luck. Thus, typically actors and musicians tell each other to "Break a leg" before they go out onto stage to perform. Therefore "Break a leg" is an idiom meaning "I wish you good luck."

The expression is now used widely outside the theatre as superstitions and customs travel through other professions and then into common use.