What is the significance of the gifts Boo Radley leaves in the knothole?

In the early chapters of the book, Jem and Scout find several small items, ranging from sticks of gum to a pocket watch, left by Boo Radley in the knothole of a tree on the Radley property. These gifts are the first of several kindnesses that Boo extends to the children, ultimately culminating in Boo killing Bob Ewell to protect Jem. The gifts also represent one of the ways that Boo tries to engage with the world around him without giving up the secrecy and privacy that he requires. Despite his reclusive nature, Boo engages the Finch children in a more generous and kind way than many of the other adults that they encounter. But because of Boo’s limitations, his interactions must take a remote form.