At the end of the novel, Scout and Jem are attacked by Bob Ewell, who is trying to hurt Atticus for making him look like a fool in court. Boo saves the children, and even carries Jem home.
Throughout the novel, Boo had been like a ghost—there were only rumors about his misdeeds, though the children had never seen him. At the end of the novel, Scout realizes that Boo does not want to be publicly recognized for saving the children. Instead, he wants to remain alone in his home, away from others and away from attention.
Atticus, earlier, had told Scout and Jem that it was a sin to kill a mockingbird, because they don't hurt anything but only sing for others' pleasure. Scout realizes that Boo is the mockingbird. He saves the children not for recognition but because it was the right thing to do. Scout tells Atticus: “Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?” This means that to share Boo's deed with others would prevent Boo from living the life of solitude he wants. Scout shows maturity in recognizing that Boo is happy to be alone.