This conversation comes as Scout and Jem think about their experiences. The exchange between the siblings is as follows:
You know something, Scout? I've got it all figured out, now. I've thought about it a lot lately and I've got it figured out. There's four kinds of folks in the world. There's the ordinary kind like us and the neighbors, there's the kind like the Cunninghams out in the woods, the kind like the Ewells down at the dump, and the Negroes.
Jem is attempting to understand the prejudice he sees surrounding him. His division of people into four groups accounts for the hatred and discrimination he witnesses everyday. He still willingly believes that people are different, and can be categorized according to race or social class.
But Scout has a different viewpoint. Her hierarchy only has room for one group. She replies "Naw, Jem, I think there's just one kind of folks. Folks". This reveals her own understanding of the world around her. She is not attempting to rationalize prejudice; she is merely accepting that it exists. This allows her to see that all people are one, no matter their background. But Jem doesn't agree:
"That's what I thought, too," he said at last, "when I was your age. If there's just one kind of folks, why can't they get along with each other? If they're all alike, why do they go out of their way to despise each other? Scout, I think I'm beginning to understand something. I think I'm beginning to understand why Boo Radley's stayed shut up in the house all this time . . . it's because he wants to stay inside."
Jem is worried by Scout response, because it basically means that there's no logical reason for discrimination based on race or class. This contrasts with his view of the world as a logical place. It's rather ironic that Scout has a better grasp on humanity, due to her age and relative lack of maturity.
Jem is trying to map out and make sense the socio-economic classes that make up their rural town of Maycomb, and the surrounding environs of Maycomb County, Alabama. From his thirteen year old point of view, he saw the community divided into 4 groups.
"There's four kinds of folks in the world. There's the ordinary kind, like us and the neighbors, there's the kind like the Cunninghams out in the woods, the kind like the Ewells down at the dump, and the Negroes."
In "white" society there are differences in rank and pecking order. Jem sees caucasians divided by familial lineage, financial, educational and social status, putting the suburban dwellers of Maycomb, like themselves and their neighbors, at the top of the hierarchy, and the poorer, less educated, but hard working, farmers or laborers like the Cunninghams in the middle of that group. The "white trash" or the most impoverished, uneducated and perhaps most socially backwards group of white folks, such as the Ewells, reside at the bottom of the white society strata.
In the segregated South of that era, the Negroes, regardless of wealth, education, or career were beneath - and separate- from the Caucasian hierarchy. Racism separated people of color physically, socially, educationally, financially, and in some peoples frame of reference, even morally. People of color were considered totally and wholly separate, and definitely not equals, in Maycomb society.
Jem may have left out the groups that Scout mentions, "the Chinese and Cajuns" partly because they weren't in the vicinity, but also partly because they were regarded as "people of color" and could simply be lumped in with Negroes, because they were not white and therefore not part of the hierarchy of Caucasians. Aunt Alexandra sees the differentiation in rank and class among white folks as having to with literacy. Those who can read and write are "above" those who are illiterate.
In a strange irony, young, innocent, clear-eyed Scout actually agrees with her aunt. The difference is she doesn't look down on those who are less educated. Scout sees the difference between white people as really only hinging on one thing - opportunity and access to education. She sees no difference between the Finches and the Cunnighams.
"No, everybody's gotta learn, nobody is born knowin'. That Walter's as smart as he can be, he just gets held back sometimes because he has to stay out and help his daddy. Nothin's wrong with him. Naw, Jem, I think there's just one kind of folks. Folks."
This implies that Scout sees everyone in the white community equally...but differentiates their social status based on their opportunity to access and participate in education. We might surmise that Scout's world view could go further, and that anyone educated and literate would be in the same category, regardless of race, or class. It's not likely that she saw it this way given racist and segregated society she lived in.