Scout diffuses a potentially dangerous situation by talking to Walter Cunningham Sr. about ordinary, everyday matters.
Mr. Cunningham is the head of a drunken, angry mob that's descended on the local jail with the express intention of lynching Tom Robinson, who's being held there. Although Tom is wholly innocent of any crime, that cuts no ice with the lynch mob; as far as they're concerned, he's guilty simply by virtue of being a Black man accused of raping and assaulting a white woman.
When the mob shows up, Atticus is sitting out in front of the jail, reading a newspaper. Unbeknownst to him, Scout has made her way to the jail, concerned as she was that something was up. It isn't very long before the situation becomes intense, with the threat of violence in the air.
But Scout's able to take the heat of the situation by talking to Mr. Cunningham like he's a human being. Instead of pleading or begging with him to leave, she talks to him as if she had just met him in the street.
She talks about the legal work that her father has done for him, and about Mr. Cunningham's son, Walter Jr., with whom she goes to school. All this normal talk, coming from a child, disarms Mr. Cunningham, and the lynching is called off.