In this chapter Jem and Scout are having a long discussion with Atticus about the jury from Tom Robinson’s trial and the people of Maycomb. They are showing some development in their ability to think and question the way things are.
A page before the quotation you are concerned with, Jem has just said to Scout:
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.
Scout objects to this classification of people. She is espousing the idea that everybody is born basically the same and has to learn about life themselves. When she says “I think there’s just one kind of folks,” Jem says that’s what he used to think too. He is showing that his ability to think is developing. He is being logical when he asks,
If they’re all alike, why do they go out of their way to despise each other?
Finally, at the end of the chapter, Jem shows real progress in his view of the world with his comment about Boo Radley. The children have assumed for the entire story that Boo stays in the house because he is some kind of monster. Now, with his increased life-experience, Jem has a different view.
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.
Throughout the novel, Jem's maturation is essentially the stripping away of his innocence. Both children have been raised with such a strong sense of right and wrong and have not yet had to face the fact that sometimes the wicked win and the good suffer. Atticus has provided a role model in which he treats everyone, regardless of social status, with dignity. But Jem is learning that justice and fairness are not always a given. He begins to understand the reality that society can be cruel to those who are different and, while he rails against the injustice when Tom is convicted, he has acquired enough maturity to process this reality; while Scout is still a few years away from being able to do so. She still believes what she's learned from her father.