The novel means different things to different people, but generally it is seen as a "growing up" story with a strong message against racism, intolerance, and cruelty. I think you must be asking about the phrase itself, "to kill a mockingbird," which becomes the title of the book.
This is a reference to an incident in the novel when Jem gets an air rifle and Atticus lays down the rules. He tells Jem that he can shoot certain birds, but he is not to kill mockingbirds because they are harmless--in no way destructive--and bring only beauty into the world with their song. This becomes a motif in the novel and helps develop its themes.
Several characters have been interpreted to be "mockingbirds" according to Atticus' explanation, primarily Boo Radley. Boo is innocent and essentially helpless. He harms no one. At the conclusion of the novel, Scout makes the connection between Boo and mockingbirds. When Heck Tate refuses to bring Boo into the glare of publicity, Scout observes that "it'd be sort of like shootin' a mockingbird, wouldn't it?"
Sometimes Scout and Jem are interpreted as being mockingbirds in that they are innocent and harmless yet brutally attacked by Bob Ewell in a despicable act of drunkenness, but the mockingbird motif is developed mainly through Boo Radley's character.