Atticus says that Scout helped the mob come to its senses by reminding them that they were human.
Atticus knows that many of the people of Maycomb do not approve of him defending Tom Robinson. To them, Robinson is a black man who raped a white girl, and presumed guilty.
A group of men, including Walter Cunningham, Sr., gathers around Tom Robinson’s cell one night. Atticus knew they would be there, and set up a chair and a lamp outside Robinson’s door.
Scout, Dill, and Jem found out where Atticus was, and came down to see what was going on. Scout did not realize the significance of the mob. She felt that the situation was awkward, and tried to drum up a conversation with Cunningham because she was friends with his son.
"Entailments are bad," I was advising him, when I slowly awoke to the fact that I was addressing the entire aggregation. The men were all looking at me, some had their mouths half-open…. (Ch 15)
Cunningham is embarrassed and confused, and leaves—taking the mob with him. Atticus tells Jem that they would not have killed him because a mob is “always made up of people, no matter what” (ch 16). He chuckles to state that what reminded them of this was Scout talking to Walter.
"That proves something- that a gang of wild animals can be stopped, simply because they're still human….” (ch 16)
This incident demonstrates that no matter what happens, the people of Maycomb are still friends and neighbors. Tensions are high, but they have to stick together no matter what. It took Scout’s innocent intervention to remind Walter Cunningham of that.