What is Scout's secret?

I'm thinking you are referring to the tree at the edge of the Radley house. There is a knot in this tree and inside there is a gift. At first it is chewing gum subsequently she finds scrubbed and polished pennies, carved miniatures (of Jem and Scout), twine...Scout does tell her brother Jem. Both decide to keep it a secret(especially from Atticus). Although Scout is pretty sure that they were meant for her, there is doubt in her mind if she should accept them. Of course Boo is reaching out to both kids and, in the end, the secret really becomes symbolic of a sort of friendly pact between the mysterious Boo and the kids.