When Scout and Jem get air rifles for Christmas, Atticus tells them, “Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird.” Scout does not understand what her father means, and this is first time he has ever spoken of anything as being sinful. She decides to ask Miss Maudie, and Miss Maudie explains, “Mockingbirds don't do one thing but make music for us to enjoy.” They never cause any harm; they just “sing their hearts out for us.” Something that is so beautiful and so generous and so peaceful should not be harmed.
The mockingbird is mentioned one more time, much later in the novel. Scout and Jem have been attacked by Bob Ewell, and Boo Radley comes to their aid, killing Ewell in the process. At first, Atticus thinks that Jem might be responsible for Ewell's death, but when Mr. Tate finally makes Atticus understand what really happened, Atticus agrees to let Mr. Tate's explanation stand. Ewell fell on his knife. Period. Mr. Tate is completely unwilling to put Boo Radley into the limelight. If the town were to realize that he is the one who saved Scout and Jem, they would make a fuss over him, wanting to do all kinds of favors for him, thinking they are doing him a service. The reality would be the opposite. Boo Radley would be overwhelmed with “kindness” and miserable.
After Mr. Tate leaves, Atticus asks Scout if she understands about Ewell falling on his knife. She tells him that she does and that Mr. Tate is right. Then she adds, “Well, it'd be sort of like shootin' a mockingbird, wouldn't it?” In her childhood innocence, Scout understands that Boo Radley is like a mockingbird. He causes no harm; he merely wants to brighten the world in the limited ways in which he knows how. He is, beneath his reclusive exterior, beautiful and generous and peaceful, so he should not be harmed.