The mob consists of a group of white men who share very negative racist views. They are apparently not the pinnacle of Maycomb's society. This is made clear when Scout comments:
There was a smell of stale whiskey and pigpen about,
The men were either farm laborers or small-time subsistence farmers who tended to drink or who have now drunk to give them courage for what they intend doing. Mr Cunningham's presence, as well as the reference to the decrepit condition of the men's vehicles, support the idea that they are definitely not the best Maycomb society has to offer.
The men's prejudice leads them to believe that it is an abomination that Tom Robinson should have violated a white girl, Mayella Ewell. In their minds, the mere fact that Mayella is white suggests that she was an innocent victim and could not possibly have been a willing party to what had occurred between her and the accused. In their minds, Tom, being black, must be guilty. These men do not care about justice or a fair trial. Their minds have been made up and they have come to the jailhouse to exact punishment. Their intention is to remove Tom from his cell and execute him.
The fact that the men are in a group empowers them and strengthens each man's resolve to see, what he believes, is justice about to be done. The group is driven driven by a skewed mentality that cannot and will not see reason. Their belief that whites are superior and their hatred for those not of their kind is what drives them.
Scout recognizes Mr Cunningham in the group and starts talking to him. She tells him about his entailment and about knowing his son, Walter, and how she and Jem had invited him over for dinner. Her recognition and familiarity obviously unnerves him and he later responds to her by saying that he will extend her greeting to his son. He then tells the group:
Let's clear out. Let's get going boys.
The men then shuffled back to their dilapidated cars and left.
Scout's intervention resolves the stalemate and probably makes Mr Cunningham realize the foolishness of their purpose. He obviously feels guilty and uncomfortable about wanting to commit a heinous act in the presence of someone as innocent as Scout and the other two children, Jem and Dill. Also, the fact that she recognized him and spoke so freely about her father helping him with his entailment and her association with Walter accentuates his discomfort and essentially pushes him to give up performing what would clearly have been a malicious deed.
Tom Robinson has been arrested for the alleged rape of Mayella Ewell and is in jail. Atticus sets up a chair and lamp outside the courthouse jail to protect Tom. Atticus knows that there is a possibility that a lynch mob will form to forgo a trial, and he is right. Scout follows Atticus to the courthouse because she, too, feels that something is going on. When the lynch mob approaches Atticus, he attempts to talk them down. Scout steps in and asks Atticus what is going on. She also sees Mr. Cunningham, Walter’s father, in the crowd. Scout says, “Hey” to Mr. Cunningham and tells him that she goes to school with Walter.
Scout diffuses the angry mob first because she is an innocent in the situation who reminds the other men of their own children. Also, by Scout pointing out Mr. Cunningham, she successfully reduces the mob to a group of individuals. Alone, the individual men would not have been there, but as a mob, they gain power from each other. Scout successfully puts down the possible lynching without knowledge of what the men are doing, yet she is able to play upon their emotions by being a young, innocent girl who makes them feel ashamed of their actions.