The note from the kids to Boo asked him if he would like to come out of the house and have some ice cream with them.  This occurs shortly after Atticus puts a stop to their play; they--Jem, Scout and Dill--had been working hard on writing, producing and acting out a little melodrama based on what they imagined the Radley family's activities to be.  They also found out about this time from Miss Maudie that Boo Radley had been a very nice boy when he was young, and that she believed he had been the victim of a harsh, "foot-washing" religious tyrant of a father.