What does Scout learn about mob mentality in To Kill a Mockingbird?

Scout learns that mobs are an unthinking herd of people moving on solely one idea.

In Chapter 15 it is clear that tensions run through Maycomb's streets and outlying areas. Scout and Jem overhear some of the businessmen and the sheriff who have come to the Finch home in order to suggest that Atticus obtain a change of venue. Then, too, there is the threat of the Old Sarum bunch with which they are concerned.

Later that evening at almost ten o'clock, Jem worries about his father and heads to town; Scout tags along, of course. As they walk up the sidewalk, they see their father sitting before the jailhouse door where a single light burns. Soon, the children notice four dust-covered vehicles approach. Men with hats pulled down low over their foreheads get out. One of the men says, "You know what we want....Get aside from the door, Mr. Finch." The other men speak in near whispers and crowd together.

Suddenly, Scout recognizes Mr. Walter Cunningham, who sometimes comes to the house with vegetables with which he pays Atticus for services. His son attends school with Scout, as well. "Hey, Mr. Cunningham," Scout candidly says. Having singled Mr. Cunningham from the mob, Scout has caused Mr. Cunningham to feel embarrassed for his aggressive attitude toward Atticus, who has always been kind to him. In addition, she has individualized him and causes him to feel uncomfortable.

Scout's friendly overture of addressing Mr. Cunningham has removed him from the mob. She has effectively destroyed the singleness of thought which has propelled the men forward; now, they become uncomfortable and realize that their identities are not a safe, singular one. Mr. Cunningham bends down and speaks to Scout; then, he says to the others, "Let's clear out." 

Later, in Chapter 16, Atticus explains that the previous night Mr. Cunningham was part of a mob, but "he was still a man." Scout recalled this recognition of his individuality for Mr. Cunningham. Atticus tells Scout and Jem, 

...you children last night made Walter Cunningham stand in my shoes for a minute. That was enough.

"That was enough" to break the singleness of thought, the one idea, of a mob.