I would suggest a slightly different translation than Andrew's. "Break a leg" was originally used in the context of theater to mean "Good luck!" According to theater superstition, saying the words "good luck" is actually considered bad luck. So instead of wishing a performer good luck, it is customary to to say, "Break a leg."

"Break a leg" means to go ahead and try something. It's generally used to tell someone else to proceed with something that is, in the eyes of the speaker, not worth doing or dangerous.

It's a way of saying Good Luck, in an intentionally opposite way. Wikipedia can explain it better than me. "Break a leg" is an idiom in theatre used to wish a performer "good luck" in an ironic way. Well-wishers typically say "Break a leg" to actors and musicians before they go on stage to perform. The origin of the phrase remains obscure. The expression reflects a theatrical superstition in which wishing a person "good luck" is considered bad luck. The expression is sometimes used outside the theatre as superstitions and customs travel through other professions and then into common use.