Boo Radley, who's always been this strange figure of legend to them, is trying to reach out to them. The knothole is Boo's sole means of communication with the outside world; it's also the only way he can reveal something of himself and his true personality to the Finch children. All the various items he leaves in the knothole provide a tantalizing glimpse into his own little world, a world that no one else has ever had the chance to see.
