To Kill a Mockingbird

The gifts in the tree are a key turning point in the novel, because we realize something very important about the mysterious character Boo Radley. Prior to the gifts in the tree, we all think that Boo is a "malevolent phantom" intent on committing small crimes and peeping in people's windows at night. Rumors of scissors, creepy nighttime hauntings, and enigmatic family feuds all circulated around Boo Radley; he is feared by the townsfolk and children, and used as the token Boogey Man to blame everything on. And, the summer of the opening of the novel, Jem, Scout and Dill's lives are taken up with attempting dares all revolving around Boo and his house.
The gifts in the tree show us a different side of Boo Radley; he was the one to put those gifts there. They show that he probably isn't some evil man intent on causing harm in the town, but rather, a lonely guy who really just wants a friend. And, since he can see the children playing from his house, he knows Jem and Scout pretty well by now. He thinks their escapades are amusing and funny (as evidenced by the laughter when Scout fell out of the tire), he cares about the children deeply and covers for them (as evidenced by the sewn-up pants that Jem left behind on the fence), and wants to show them that he is there, and that he is a friend. So, he puts gifts in the tree, one being little carved figures that look very much like the two children.
Another significant aspect to the gifts in the tree is that it provides an opportunity to see just what a cruel family situation Boo lives with; once his brother discovers he has been putting gifts in the tree, he fills the tree up and stops that act of kindness right in its tracks. No wonder Boo is unhappy and lonely, and reaching out--he lives in a pretty sad home.