In the Harper Perennial Modern Classics edition of To Kill a Mockingbird, Atticus speaks his famous line on page 103, just a few paragraphs into the novel's tenth chapter.
Scout and Jem have received air rifles for Christmas, and they are anxious to practice their shooting. Atticus would rather...
they shoot at tin cans, but he knows his children. He tells them to shoot bluejays if they want (and if they can hit them) but reminds them that “it's a sin to kill a mockingbird.”
Scout doesn't know what her father means. She has never before heard him say that anything is a sin, so this statement surprises and confuses her. She goes to Miss Maudie for an explanation. Miss Maudie tells her, “Mockingbirds don't do one thing but make music for us to enjoy.” They don't ruin gardens or make inconvenient nests. They just “sing their hearts out for us.” They are innocent, and they fill the world with beauty. No one should kill a creature like that. In fact, it is an act of wickedness to do so.
After Miss Maudie's explanation, Scout understands to a point, but Atticus's words don't really reach deeply into her mind until Boo Radley rescues her and Jem from Bob Ewell, killing Ewell in the process. Atticus and Mr. Tate decide that they will simply say that Ewell fell on his knife. They do not want to expose Boo to the attention that he will receive if they reveal his part in the incident. They realize that the community's gratitude will overwhelm Boo. Atticus asks Scout if she understands. That's when the idea of the mockingbird rises to the top of her mind. “Well, it'd be sort of like shootin' a mockingbird, wouldn't it?” Scout asks her father. Boo Radley does what he can to spread beauty throughout the world, even though he is a recluse. Recognition and attention would kill his spirit just as surely as a bullet from an air rifle would kill a mockingbird.