Miss Maudie helps to ground the children's unrealistic fantasies regarding Boo Radley in Chapter 5. After Scout asks Miss Maudie if she thinks Boo Radley is still alive, Maudie first answers by correcting Scout that his name is Arthur and that of course he's alive because he hasn't been "carried out" yet (Lee 48).
Maudie then talks with Scout about the real reasons that Boo stays inside. She starts with, "Arthur Radley just stays in the house, that's all... Wouldn't you stay in the house if you didn't want to come out?" (Lee 49). She later explains that Mr. Radley was a "foot-washing Baptist" and that "the Bible in the hand of one man is worse that a whiskey bottle in the hand of [a good man like Atticus]" (Lee 49-50). Maudie suggests that the Radley family's strict definition of sin and their wish to limit Arthur from coming into contact with temptation might factor into why he is shut away.
Finally, Maudie ends on a note of uncertainty. She says, "I remember Arthur Radley when he was a boy. He always spoke nicely to me, no matter what folks said he did... The things that happen to people we never really know. What happens behind closed doors, what secrets--" (Lee 51). As Maudie and Scout shift their conversation back to Atticus, Maudie has shown Scout why the rumors about Boo Radley have grown and persisted. Since people don't know the full story about Arthur, they've made it up. But, the reason Boo stays inside, according to Maudie, likely has to do with his strictly religious parents and his family's private habits about which we can't fully know.