In Chapter 15, Atticus travels to the Maycomb jailhouse to sit out front of Tom Robinson's cell as a precaution. Jem and the children decide to look for Atticus and spot him outside of the jailhouse. Suddenly, several cars arrive from the Meridian highway, and a group of men surround Atticus after they get out of their vehicles. The group of men are poor farmers who reside in the northern part of the county, known as the Old Sarum Bunch. Walter Cunningham is the group's leader, and they travel to the Maycomb jailhouse in an attempt to lynch Tom Robinson. Fortunately, Atticus refuses to leave the scene, and Scout's presence is enough to make Walter rethink his actions. The Old Sarum bunch leaves without harming Tom Robinson. 

The people who come to meet Atticus at the jail are definitely there for trouble.  They are coming to meet Tom Robinson, because they want to skip the trail altogether and just kill him.  They are a lynch mob.

The men arrive in “four dusty cars” and there are quite a few of them, because they are described as getting out “in ones and twos” to confront Atticus.  Atticus addresses one of the as Walter Cunningham, so we know that he is one of the leaders of the mob because he addresses Atticus.  This is Walter Cunningham, Sr. not Scout’s friend.  We also know that the rest of the men smell like whiskey and pigpen, and that at the fact that they are dressed in “overalls and denim shirts buttoned up to the collars” makes them farmers. Scout does not know them.

Scout, Jem and Dill add a unique twist to the situation.  The presence of children at a lynch mob does not make sense.  Scout diffuses the situation by jumping right into the middle of the mob and talking to Mr. Cunningham.  Everyone is completely shocked, but then Mr. Cunningham comes to his senses and he leaves, taking the crowd with him.