Scout is very curious about why Boo Radley stays inside. She and the other children try to make him come out.
Nobody knew what form of intimidation Mr. Radley employed to keep Boo out of sight, but Jem figured that Mr. Radley kept him chained to the bed most of the time. Atticus said no, it wasn’t that sort of thing, that there were other ways of making people into ghosts. (ch 1)
Atticus’s words are very relevant here. The citizens of Maycomb think Boo is crazy, so he stays locked up. He has been forced to become “a ghost” because he is different.
Miss Maudie explains to Scout that “sometimes the Bible in the hand of one man is worse than a whiskey bottle in the hand” of another. (ch 5) In other words, Mr. Nathan Radley is one of the causes of Boo’s mental distress and the reason he stays inside.
It is actually Jem who connects Boo’s isolation with prejudice.
I think I’m beginning to understand why Boo Radley’s stayed shut up in the house all this time… it’s because he wants to stay inside.” (ch 24)
This is a significant point. Boo is one of the story’s mockingbirds, or innocent victims of prejudice. He suffers not because of the color of his skin, but because people don’t understand him. He has been oppressed all of his life, not by society, but by his father’s religious zeal.