From the passage around page 68:

Harper Lee creates sympathy for Boo Radley and uses him to show the beginning of adolescence in jem. Boo Radley (or as is suspected) uses the hole in a tree to communicate with the outside world. As he has been kept in for so long the whole of Maycomb has a bit of a vendetta against him and use him as a scapegoat even for when the weather is bad! As it has been so long for him to meet anyone else it feels cruel when the tree is filled up and he can no longer. Jem has started to become of an age where events don’t just pass by, but he actually feels emotional about it. We can see this because after he is coming back with Scout he stays outside and cries for a little.