Atticus means that Scout helped Walter Cunningham realize that Atticus was just a person.
Atticus does his best to try to teach Scout empathy. He tells her that she will get along with a person better if she is able to “climb into his skin and walk around in it” (Ch. 3). Scout learns this lesson well. When Atticus’s client is threatened because he is a black man accused of raping a white woman, he decides to sit outside the jail in case anyone comes. Scout and Jem see what he is doing and go to see what is going on.
Walter Cunningham and his gang do come for Tom Robinson. However, Scout walks up to Walter and begins talking to him.
“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)
Walter is so surprised that he stops, and they leave. Atticus sees this as a good sign. To him, they are not an angry mob. They are just a group of men.
“So it took an eight-year-old child to bring ‘em to their senses, didn’t it?” said Atticus. “That proves something—that a gang of wild animals can be stopped, simply because they’re still human. …” (Ch. 16)
Scout is just a child, but she is an astute one. She was worried about her father, and she acted. She did not comprehend enough of the situation to be afraid. She just tried to be polite to the father of her classmate. It worked. It reminded Walter Cunningham of the stakes. Scout woke him up to what he was about to do and the fact that it was wrong.
The trial of Tom Robinson puts the entire town on edge. As Robinson's attorney, Atticus is a lightning rod for all of the hatred and resentment. He shows his humanity by protecting his client at all costs. In this incident, Scout proves that she is her father's daughter.