Aunt Alexandra thinks that the Cunnhinghams are trash, and therefore will not let Scout play with him.
Scout asks to play with Walter Cunningham, Jr., but Aunt Alexandra says “Indeed Not.” Scout asks her why she can’t play with the boy. After all, this was the same boy that Jem invited to lunch when Scout was in first grade, and Atticus chatted with him like an adult.
"I'll tell you why," she said. "Because- he- is- trash, that's why you can't play with him. I'll not have you around him, picking up his habits and learning Lord-knows-what. You're enough of a problem to your father as it is." (ch 23)
Jem has to drag Scout away “sobbing in fury.” She is upset that Aunt Alexandra groups Walter in the same category as the Ewells. She does not consider them anything alike, and neither does Atticus. Scout knows that the Cunninghams are decent people, even if they are poor. Jem tells Scout that he has it all figured out.
“…The thing about it is, our kind of folks don't like the Cunninghams, the Cunninghams don't like the Ewells, and the Ewells hate and despise the colored folks." (ch 23)
Everybody needs somebody to look down on. There is nothing wrong with the Cunninghams, but they don’t have money and the people who are from good families look down on them. The Cunningham’s can look down on the Ewells because they do live in a much rougher way.
Scout has learned that people should be judged not by their class, but by their character. This view of people she got from Atticus, who did not instill class-based values on his children like Aunt Alexandra wanted him to.