That phrase comes from the book To Kill A Mockingbird by Harper Lee, and it literally means to kill a mockingbird. In the book, Atticus Finch tells his daughter Scout that it is a sin to kill a mockingbird, because all it does is sing for people to enjoy and it never hurt anybody. In the book, the mockingbird represents good, innocent people who are destroyed by the evil in the world. It’s a moral imperative to protect the helpless and the innocent.