Atticus tries to help Jem and Scout understand that what people do in their own home is their own business, and to think about what they would feel like if someone was trying to spy on them. He says that the wouldn't like it if he just barged into their rooms, and that they might get a lot farther if they just knocked on Boo's door and asked to try and get to know him. It is a good point and makes a lot of sense; people definitely are entitled to their own privacy and shouldn't be imposed on.
