When Scout first begins school at the start...
In Harper Lee's To Kill a Mockingbird, Aunt Alexandra does not mince words with regard to how she feels about the Cunninghams. She says they are not people with which the Finches associate, and this especially confuses young Scout.
When Scout first begins school at the start of the novel, she has a disagreement with Walter Cunningham because she feels he had gotten her "off on the wrong foot" with her new teacher. At recess Scout is ready to beat him up, but Jem stops her and invites Walter home to have lunch at the Finch house. Scout gets in trouble for pointing out Walter's unusual behavior at the dinner table. She recalls the encounter later in chapter twenty-three, while having a family discussion that begins about Bob Ewell and moves on to the Cunningham family. It is clear, and not surprisingly, that Aunt Alexandra is not as charitable toward Maycomb's lesser folks as Atticus is. While Atticus defends the Cunninghams, noting that once you have them in your corner they always have your back, Alexandra sees things much differently. For example, she says:
Jean Louise, there is no doubt in my mind that they're good folks. But they're not our kind of folks.
Scout recalls her aunt's refusal to allow her to visit Calpurnia's home. Even in her innocence, Scout notices a similarity between that situation and the one she faces now:
This time the tactics were different, but Aunt Alexandra's aim was the same.
So, Scout questions Alexandra, wondering why, if the Cunninghams are good folks, she cannot be nice to them. Her aunt insists there is no difficulty in being nice:
You should be friendly and polite to him, you should be gracious to everybody, dear. But you don't have to invite him home.
It is relatively easy to see that Aunt Alexandra's social values allow one to be civil and "gracious" to someone from the same town. But someone who is not considered a social equal is, in her mind, to be kept in his or her place; one that does not intersect with her place. She points out that the Cunninghams are in no way related and also that no Finch woman would ever be interested in a Cunningham man. And the only way a Cunningham will be allowed at their house, according to Scout's aunt, is if he comes to see Atticus on business.
Scout points out that she wants to play with Walter, and asks why she cannot:
She took off her glasses and stared at me. "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."
Before Scout can react (physically, it would seem) to Aunt Alexandra's edict, Jem steps in and leads her sobbing from the room.
In the same way that Scout cannot see anything wrong with Dolphus Raymond or Tom Robinson, she does not understand the harm in playing with a youngster that is, even by her aunt, deemed from a family of "good folks." The irony, of course, is that at least one Cunningham (on the jury) can see beyond race to Tom Robinson's innocence, while Alexandra's social equals possess no tolerance, or even sympathy, for the wrongs done to the Robinson family.
This incident is at the core of the struggle regarding the racial divide in the imaginary Maycomb, as well as the South in general, at that time. It reflects the battle with and the sometimes disheartening outcome in, as Atticus puts it, "the secret court of men's hearts."