In Chapter 15, a group of town men come to the jail in a mob. Their plan is to go inside, get Tom Robinson, and lynch him. Atticus is the only one who stands in their way. At first, it doesn't look good. The mob is fired up and ready to attack.
However, the kids - Scout, Jem and Dill - step in. They have gone to the courthouse against orders, and fearing for Atticus, they step out into the crowd. This surprises the men, but it isn't until Scout mentions Mr. Cunningham by name, and tries to remind him that she goes to school with Walter, that the spell of the mob is broken and they leave.
This scene is intended to show not only the nature of the community, but also the nature of a mob mentality. People in a group can't always think for themselves. Individually, most of these men are good men - as a group, however, they are swayed by social attitudes and standards, such as racism. This is proven by the jury's vote. Atticus acknowledges that the jury knows that Tom is innocent - but because of social attitudes and tradition, they can't vote against the word of a white man (Bob Ewell).
In Chapter 15 of To Kill a Mockingbird, Jem and Scout come to the aid of their family by diffusing the tensions generated by the Old Sarum group that accosts Atticus.
After Sheriff Heck Tate and business leaders come to the house in order to urge Atticus to obtain a change of venue for the Tom Robinson trial, Jem worries; so he asks his father about them. The next day is Sunday, but Scout and Jem notice that there are people present who usually do not attend. For instance, Mr. Tate is there, and he never goes to church; Mr. Underwood also is present, and he, too, never attends. "Something must have brought Mr. Underwood out," the children think.
So, when Atticus takes the car and drives to town late that evening, Jem decides to go himself and ascertain if his father is safe. When they near the jailhouse, the children hear their father as he is accosted by men who tell him to move away from the jailhouse door. Atticus does not do so, and he warns the men that Sheriff Tate is there. But one man counters that Tate is in the woods called "off on a snipe hunt."
At this point, Scout rushes forward, greeting her father, whose face reveals "a flash of plain fear." Jem and Dill advance, and Atticus tells them to go home, but Jem shakes his head. Angered at the treatment of her brother, Scout kicks a man higher than the shin at which she has aimed. Another man growls to Atticus to get the children out of there. But before this happens, Scout sees Mr. Cunningham, and she greets him personally, telling him to say hello to Walter and asking about his entailments.
Suddenly, a change comes over Mr. Cunningham as he remembers how kind Atticus has been to him, and he then considers what he and the others are contemplating as an action. Finally, Mr. Cunningham bends down and tells Scout he will tell Walter that he has seen her. He calls off the other men, "Let's get going, boys."