Chapter 15 in To Kill a Mockingbird starts off with Sheriff Heck Tate paying a visit to Atticus. Jem and Scout are surprised to see a bunch of men wanting to speak to Atticus, and Atticus tells Jem to let the men come on in, but Jem tells him they want to talk to him outside. Scout is interested in what the men have to say to Atticus.
In Maycomb, grown men stood outside in the front yard for only two reasons: death and politics. I wondered who had died. Jem and I went to the front door, but Atticus called, "Go back in the house."
Atticus, Jem and Scout all knew that something was going on when all the men showed up at Atticus's house. Scout makes the statement about grown men usually just talking about death or politics, and she wonders who had died. What Jem and Scout don't realize yet is that Heck Tate had come to warn Atticus of a mob coming to the jail. Tom Robinson is being held at the Maycomb jail awaiting trial, and some of the men in town are mad about it and plan to take things in their own hands. Atticus is concerned for the safety of Tom, so he goes and stays in front of the jail. Jem, Dill and Scout sneak out to see what is going on, and when they see the mob of men, Scout goes running to her father.
The men are there because they are determined to get "rid" of Tom themselves. In the south at this time, some white people turned to violence in response to what they thought was a crime. Although there is no evidence that Tom is guilty, the men want Atticus to step away from the jail and let them deal with Tom the way they see fit. Without knowing it, Scout has saved Tom's and quite possibly Atticus' life.
I looked around the crowd. It was a summer's night, but the men were dressed, most of them, in overalls and denim shirts buttoned up to the collars. I thought they must be cold-natured, as their sleeves were unrolled and buttoned at the cuffs. Some wore hats pulled firmly over their ears. They were sullen-looking, sleepy-eyed men who seemed unused to late hours. I sought once more for a familiar face, at the center of the semi-circle I found one.
When Scout sees Walter Cunningham, she speaks to him and tells him that she goes to school with his son and to please tell his son she said hello. At this moment Scout has shown the most courage of all. By talking to Walter Cunningham and making it personal, Scout has made the men in the mob see the human side of the night. The men go to the jail with revenge in their hearts, but it takes a little girl to bring about a change in their hearts, if only for the night.