In this quote, Atticus shares some painful truths with his children. Jem and Scout have seen racism in all its ugliness during the unfair trial of Tom Robinson, and they now learn from their father that prejudice and racism is a huge part of life, both in the courtroom and far beyond.

Jem and Scout are in a state of disbelief that Tom has been found guilty of the rape of Mayella Ewell. They have witnessed the compelling courtroom testimony in Tom's defense, and they know beyond a shadow of doubt that he is innocent. In the context of their confusion, Atticus explains the nature and prevalence of racism.

In the first part Atticus's talk with his children, he reveals that “white men cheat black men” on a regular basis. In saying this, he alerts his children to the plight of Black people both in Maycomb and in America as a whole. In the second part of his statement, he tells his children in no uncertain terms that any white man who practices racism and takes advantage of Black people just because he can is “trash,” regardless of his station in life, how much money he has, or if he comes from a “respectable” family.

This quote comes from Atticus Finch in chapter 23 when he discusses the outcome of the Tom Robinson trial with his children. Jem expresses his deep misgivings regarding racist juries and cannot fathom how rational jurors would ever convict Tom Robinson without evidence. Atticus acknowledges that the jury carried their "resentments" into the courtroom but tries to explain that racism exists on smaller levels. Atticus's quote means that racial discrimination happens on an everyday basis, and he encourages his children to recognize the various ways in which Black people are cheated in their society.

After witnessing racial injustice firsthand, Jem and Scout lose some of their childhood innocence and realize for the first time that Black people are not treated fairly. This quote is meant to increase Jem and Scout's awareness and remind them that racism not only exists in Maycomb's courthouse but throughout all sectors of society. Atticus's quote also underscores the importance of equality and defending vulnerable individuals, which is an important theme throughout the story. Atticus is explaining how taking advantage of less fortunate, helpless black people is immoral and unethical. But in a society founded on Jim Crow laws, taking advantage of Black people is prevalent and commonplace. Atticus's quote is meant to expose Maycomb's systemic racism and make his children aware that racism is reprehensible and exists outside the courthouse.

Tom's trial is a pivotal moment for Jem and Scout. They listen to the testimonies and understand Tom's innocence, so they are shocked by the jury's ruling. They don't understand how the jury can get away with such an overtly racist decision. They see how society's racist attitudes can lead to a black man's death.

Atticus tries to explain to the children that the jury can get away with a decision like this because of racism. In this quote, he explains to them that racism happens every day. The trial is a very big and obvious example of a black man being cheated by white men, but Atticus reminds us that it happens every day in smaller ways. Now that Jem and Scout's eyes are more open to racism, they will see this cheating more.

While To Kill A Mockingbird is mostly about racism, the novel does touch upon class differences, with examples such as the Cunninghams. In the south, many people pride themselves on their wealth or their well-known family. In the second part of the quote, Atticus emphasizes that anyone can be racist, even people from high-class families. Atticus says that racists are trash, despite their other qualities. It doesn't matter where you come from; you should not cheat someone else.

The previous post is a good one. I would just add that Atticus is trying to explain that it is never a proper or moral act to take advantage of someone who is (or is perceived to be) weaker. The same reasoning would apply to someone who cheats or takes advantage of a child or someone with mental deficiencies. Taking advantage of someone from a position of strength or authority without a proper legal foundation is a reprehensible act. At the time of the novel's setting in the 1930s, black citizens often had no choice but to accept a white man's terms; refusing to do so, or publicly objecting to such an agreement, often meant dangerous repercussions for the black man.

These words of Atticus Finch are said after the trial of Tom Robinson in response to Jem's questioning of the verdict against Tom:

"You just can't convict a man on evidence like that--you just can't"

Atticus explains that the men of Maycomb can and did exactly that because despite what the U.S. Constitution demands, Tom Robinson did not have a "square deal" in the courtroom. For, the jury members carried "their resentments right into a jury box." This egregious act of the jury is what prompts Atticus's words that no matter what family a white man comes from, no matter what accomplishments a man has, if he cheats a black man of his constitutional rights, that man is "trash"; that is, he is unconscionable and lacks any dignity and character. Further, he tells the children that he is sickened by "low-grade white men" who take advantage of a Negro's ignorance.