I assume that you are talking about what happens Chapter 13. If that is what you are talking about, then the answer is that two boys join Tom Sawyer in his "life of crime." The boys are Joe Harper and Huck Finn.
Both Tom and Joe are going to run off and become criminals because they are sad. Tom is sad because Becky Thatcher has rejected him. Joe is sad because his mother has whipped him for something he did not do. Huck is not sad -- he just comes along for fun. He's sort of the local bum kid who doesn't go to school. All the parents warn their kids to stay away from him.