Aunt Alexandra tells her children that the Cunninghams are lower class.
Scout wants to be friends with Walter Cunningham, and sees no reason why she can’t be. Aunt Alexandra wants Scout and Jem to understand that as Finches, they are special. They are above what Jem calls “tacky” people like the Cunninghams.
"The thing is, you can scrub Walter Cunningham till he shines, you can put him in shoes and a new suit, but he'll never be like Jem. Besides, there's a drinking streak in that family a mile wide. Finch women aren't interested in that sort of people." (Ch. 23)
Although Jem talks about the Finches liking “fiddlin' and things like that” and Aunt Alexandra talking about the drinking streak, the reality is that she considers them inferior people.
When Aunt Alexandra talks about "that sort of people," she is reinforcing another kind of prejudice. Racial discrimination is not the only kind present in Maycomb. Aunt Alexandra and others will look down on the Cunninghams, and teach children to look down on them. That is how the cycle of hatred, misunderstand, and prejudice continues.