Aunt Alexandra believes in Fine Folks, gentle breeding, family streaks and heredity. In her mind, no other family in Maycomb matches up to the Finches, in part because Simon Finch was the first settler in the area, and no other families can trace their lineage farther back than the Finches.
She never let a chance escape her to point out the shortcomings of other tribal groups to the greater glory of our own.
As for Fine Folks, Alexandra believed, according to Scout,
... that the longer a family had been squatting on one patch of land the finer it was.
The Finch family had benefited from "several generations of gentle breeding"--a term that Alexandra never quite specified, and that Atticus gave up trying to explain to his children. But Alexandra was convinced that all families--aside from the Finches--had some sort of streak:
A Drinking Streak, a Gambling Streak, a Mean Streak, a Funny Streak.
Alexandra seemed to believe that most anyone who was not Fine Folks or was not brought up with gentle breeding or who had a Family Streak qualified as trash. Certainly, in her mind, Walter Cunningham Jr. qualified. The Cunninghams suffered from a Drinking Streak and, more importantly,
"... you can scrub Walter Cunningham till he shines, you can put him in new shoes and a suit, but he'll never be like Jem."
Alexandra never explained her definition of "trash" any further, other than to tell Scout that Walter could not come and play with her
Although Scout had heard Atticus describe any white man who took advantage of a black man "is trash," Scout had a much simpler view of people. Although Jem believed there were four kinds of folks--
"... the ordinary kind like us and the neighbors... the Cunninghams... the Ewells down at the dump, and the Negroes"
--Scout believed that there was
"... just one kind of folks. Folks."
It was her innocent way of believing that all people were really the same inside.