because they want Boo to come out so Dill and Jem can ask him a few questions. When Atticus finds out about their plan, he says that they should all stop tormenting the Radley's.
They want to give Boo a note because they want him to come out of the house and they want to know more about him, Atticus stops Jem and Dill because he doesn't want them to bug Boo.