In chapter 15, Atticus is confronted by a lynch mob, but Scout's presence influences Mr. Cunningham and the Old Sarum bunch to disperse. The next day, Atticus discusses the previous night's events with his children and defines mob mentality to explain the men's hostile emotions. He begins his definition of a mob by stating that each one consists of individuals who behave like wild animals.

The effect of mob mentality influences the members to act against their natural disposition to behave civilly. They transform into hostile people, who lose their individuality, think with a collective mindset, and respond violently. Despite their uncivil, hostile demeanor, Scout was able to remind the Old Sarum bunch of their humanity and influence them to exercise sympathy for her father.

According to Atticus, a mob is made up of people, and in Maycomb's small town, everyone is familiar with its members. By referring to them as familiar people, Atticus is humanizing members of a mob, which helps him to empathize with Mr. Cunningham. Instead of castigating Mr. Cunningham for leading the lynch mob and threatening his life, Atticus exercises tolerance and does not judge him for his transgressions.

Atticus's definition of a mob and explanation of their mentality enhances his children's perspective on their community and teaches them tolerance. He humanizes members of the mob and encourages his children not to judge Mr. Cunningham for being their leader. His definition also sheds light on the psychological effects of a mob and explains why people act differently around their peers.

Thanks to Scout, Atticus Finch has just had a very lucky escape. As he was sitting outside the jailhouse reading a newspaper, he was suddenly confronted by a drunken, angry mob led by Walter Cunningham, Sr. They'd come to the jailhouse to drag Tom Robinson out of his cell and summarily lynch him, which was not an uncommon occurrence in those days.

Thankfully, Scout turned up and started engaging Mr. Cunningham in conversation. Feeling somewhat chastened, he called off the lynching, and he and the rest of the mob left the jailhouse. Had it not been for Scout, there's no telling what would've happened to Atticus. It certainly wouldn't have been pretty.

Later on, Atticus reflects on the incident at the jailhouse. He describes a mob as “a gang of wild animals.” At the same time, however, he states that a mob is made up of humans, and it's only because of that that it's possible to stop one. In recognizing the basic humanity of the members of a mob, Atticus is showing once again his remarkable ability to put himself into other people's shoes.

He also says that every mob in a small Southern town is always made up of people you know. Hard as it may be for us to believe in this day and age, lynchings often had a carnival atmosphere in which ordinary folk would gather in large numbers to watch the brutal murder of an African American person as if it were a form of entertainment.

Following the incident involving the Old Sarum bunch outside of Tom Robinson's cell, Atticus explains to his children mob mentality. In Chapter 16, Atticus tells Jem and Scout that "every mob in every little Southern town is made up of people you know---doesn't say much for them, does it?" (Lee 97). Atticus realizes that a mob is made up of individuals with similar views. He also refers to them as a "gang of wild animals" who are still human. When a group of unruly men gets together, they have a tendency to act like a mob and lose their individuality. Mr. Cunningham is a good example of a morally upright person who succumbs to mob mentality. In a mob, people are influenced by their peers and adopt certain behaviors. Each person loses their individuality in a mob by embracing the group's attitude and perspective. Atticus essentially defines a mob as being a group of unruly individuals who adopt each other's negative mentality and act like wild animals.  

In chapter 16, of Harper Lee's novel To Kill a Mockingbird, Atticus is very specific about his definition of what constitutes a mob.

A mob’s always made up of people, no matter what. Mr. Cunningham was part of a mob last night, but he was still a man. Every mob in every little Southern town is always made up of people you know—doesn’t say much for them, does it?”

Earlier in the chapter, Atticus states that mobs do not exist in Maycomb.

“No, we don’t have mobs and that nonsense in Maycomb. I’ve never heard of a gang in Maycomb.”

Atticus, therefore, bases his definition of a mob off of one particular characteristic: "being made up of people." Therefore, Atticus is simply stating that only one thing universalizes a mob, it is simply a group of people.

Atticus does this so as to calm the fears of his children, Jem and Scout. Both are worried about the repercussions Atticus is going to face given his taking on of Tom Robinson's rape case. Both Jem and Scout are worried about backlash which they believe Atticus is going to face and Atticus calms their fears by marginalizing what a mob truly is.