In his explanations to Jem about what happened in the courtroom during the trial of Tom Robinson, Atticus Finch explains to his son Jem that there is also a private court of men's hearts in which people are often tried:
"There something in our world that makes men lose their heads--they couldn't be fair if they tried."
Atticus explains that the Jim Crow Laws make it impossible for a black man such as Tom to get a fair trial. Further, he explains that in this world in which blacks can be exploited, the uneducated and poor blacks are often cheated. But, he adds,
...whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash."
For, he is a "low-grade" man "who'll takes advantage of a Negro's ignorance." In other words, because the man realizes that he can act with impunity, he exploits the Negro who does not understand what is being done, and is easily tricked out of his money. The man who cheats the black because the social situation allows him to take advantage of him is degenerate; he is "trash" because he has no scruples.