The gifts in the tree symbolize innocence and friendship.
Boo Radley leaves a variety of gifts in the tree for the children to find, until Nathan Radley cements the hole. The children do not know who is leaving the gifts at first, but when they realize that Boo left them they know that he is reaching own. When Nathan Radley cements the hole, they realize he is preventing Boo from interacting with the outside world. Scout is worried that Boo will get in trouble when she realizes he has put a blanket on her shoulders. These events change their perception of Boo, and they begin to see him as a secret friend.