I agree with the first post but I'd like to add the the items in the knothole are Boo's attempts to make contact with the children. The first, a piece of gum, is something that every child likes and indicates Boo has thought carefully about what his first attempt at contact should be. The two Indian head pennies may have a duel meaning. First, the children could spend the money on candy ( remember the story is set in the 1930's) plus Indians were outcasts just as Boo. The third present seems most symbolic. They are images of the children carved out of soap. This may be Boo's way of telling the children "This is how I see you." This could be another attempt to get the children to see him in reality by showing them that soap images aren't particularly realistic. The yarn can be used to bind things together, like Boo and the children and the other things seem to be items Boo might have cherished in his childhood. All these things are meant to establish contact with the children but, ironically, just when the children are about to establish contact by writing a thank-you note, they find Nathan Radley has filled the knothole with cement,
The knot hole is on a tree on the Radley property. The fact that the children continually find items in there leaves open the question of WHO is placing the items there. One can assume it is Boo Radley. Not only does this prove that he does come out of his home (how else would he place the items there?), but also it could prove that he is watching the children and may knowingly be placing the items there for them. The children think perhaps they have stumbled upon someone's secret hiding place.