The gifts found in the tree are an outreach of friendship from Boo Radley to the Finch children.
The children are not sure who is putting things in the tree at first. They decide to leave a note for “whoever” is putting the gifts in the tree.
Dear sir," said Jem. "We appreciate the- no, we appreciate everything which you have put into the tree for us. Yours very truly, Jeremy Atticus Finch." (ch 7)
The children have already received another gift from Boo—Jem’s pants. Jem lost his pants on the Radley porch, and knew he would be in trouble if anyone found out, since he was not supposed to be there. Yet he found the pants returned, stitched up inexpertly. This was Boo’s attempt to keep Jem from getting punished.
In chapter 7, Scout and Jem find a ball of twine in the tree. They consider leaving it there in case it belongs to one of the schoolkids.
The children also find an old spelling medal, a knife, a watch and chain, and soap dolls carved into Scout and Jem’s shape.
Unfortunately, they visit the tree again to find Nathan Radley filling the hole with cement. He says it’s because the tree is sick, but the children don’t believe him. They are sad because their connection to their new friend is severed.
Someone had filled our knot-hole with cement.
"Don't you cry, now, Scout... don't cry now, don't you worry-" he muttered at me all the way to school. (ch 7)
Boo Radley is a very shy, lonely man. His attempts to reach out to the children show how desperately he wants friends. The children appreciate this, even before they realize what is actually going on. The gifts are Boo Radley’s first steps to coming out.