In the beginning of chapter 9 Atticus and Scout are having a conversation.

What is Atticus explaining to Scout?

When Scout puts up a question to her father, to that Atticus replied,

"It's slipped into usage with some people like ourselves when they want a common, ugly term to label somebody."

Atticus is very preventive about his kids, Jim and Scout. He didn't like his kids face the world's ugliness and does his best to protect them. To this he opens the door, explains how nasty people can be by taking the case of Tom Robbins. Atticus explains to them that doing the right thing and taking the stand for the right is difficult but results in the best.