Atticus tells Scout that "it's a sin to kill a mockingbird." Scout is unsure of his meaning, and asks Miss Maudie. "Your father's right," she says. "Mockingbirds [only] sing their hearts out for us. That's why it's a sin to kill a mockingbird." (Chapter 10)
Scout still doesn't understand what Maudie and Atticus mean. This is a demonstration of her innocence and youth. It isn't until the final chapter of the novel, when she is saved by Boo Radley, that Scout understands. Tate and Atticus agree to the lie that Bob Ewell fell on his own knife, and Scout observes that allowing the town to gain this intimate knowledge of Boo would be akin to killing a mockingbird.