Boo is the neighborhood mystery man, having not been seen outside his home since he was a teenager, when his mother promised he would no longer be a problem to anyone. The children--Scout, Jem, and Dill--listen to neighborhood gossip about him, where he presumably does things like stab his mother in the leg with scissors then go calmly back to clipping the newspaper--and think of him as a monster. However, when Mrs. Maudie's house burns down in Chapter 8, someone puts a blanket around Scout as she watches it burn; Atticus tells Scout that it was Boo who'd done it. At this point, the children begin to understand that Boo isn't the monster the neighborhood scolds said he was; he was a friend--albeit a friend they'd never seen--who left them presents and looked out for them.
After Tom's trial, Bob Ewell, having been shamed by Atticus and various people in the town, began committing small acts of revenge, such as spitting on Atticus and trying to break into the judge's home. After the Halloween festival, Ewell attacks Jem and Scout as they walk back home, and ends up dead "with a kitchen knife stuck up under his ribs" (305). Atticus thinks it's Jem, but Sheriff Tate clearly is convinced that Arthur "Boo" Radley killed Ewell to protect the children. He tells Atticus that he will say that Arthur Radley tripped on a tree root and fell on his own knife (317). When Atticus begins to argue, Tate says that he "never heard tell that it's against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he [Radley] did," but if he tells the truth, all the ladies would be bringing him cakes and "draggin' him with his shy ways into the limelight," which to Tate, is "a sin" (317).
After Tate leaves, Atticus sits down, calls Scout to him, and says, "Mr. Ewell fell on his knife. Can you possibly understand?"
Scout agrees and says, "It's be sort of like shootin' a mockingbird, wouldn't it?"
She thus refers to Mrs. Maudie's words of wisdom earlier in the book, when she and Jem got BB guns for Christmas and Atticus had told them to shoot at any birds they like, but not mockingbirds. Mrs. Maudie had said, "Mockingbirds don't do one thing but make music for us to enjoy.... That's why it's a sin to kill a mockingbird" (103). So when Scout agrees that going along with the story that Ewell fell on his knife, she helped protect Boo's privacy. Boo, being a private, shy man, had done the town a great service and didn't deserve the publicity that would go with it if word got out, making him the "mockingbird" they all decide to protect.