Where does the title of the novel come from?

The title derives from Atticus's speech about not killing Mockingbirds. He says to kill a mockingbird is a sin because all they ever do is sing and make people feel good. The metaphor extends to the misunderstood innocents in Maycomb. Both Boo Radley and Tom Robinson both try to do good things. They are falsely accused of being threats and both these gentle men, like mockingbirds sometimes are, are punished for their kindness.