Why do Dill and Jem want to give Boo Radley a note? What does Atticus say when he finds out about their plan?

The reason that they want to do this is that they are
fascinated with the idea of Boo Radley.  To them, he is like some sort of a monster.  He
is not a real person -- just someone to make up stories about and use as the basis for
games.  So they think that they can prove their bravery and daring by getting a note to
him.

Atticus makes them stop when he finds out what they
are doing.  He tells them to stop tormenting Boo.  This, to me, is because he expects
them to treat people like people -- not to use them as playthings and treat them as
monsters.