Jem and Scout find the medal in the knot hole of the tree on the Radley property. They ask Atticus about it, and he explains that it's a medal that was given out by the elementary schools for spelling bee champions.
The reader can assume that it's Boo who is leaving these gifts for the children in the knot hole. These items--the soap dolls, pennies, gum--are thing he has in his possession that he can give to the children; in essence, they are all he has. The selection of the spelling bee medal is interesting. Boo seems to want to communicate to the kids through this; perhaps he is trying to tell him that he hasn't always been a scary recluse--there was a time when he was a good student and popular.