Historically the term described any Southern white, who was too friendly with "the coloreds," especially those whites involved in interracial sex.
Of course, white trash were too poor to afford slaves. Many white trash were indentured servants from backwater parts of Scotland and Ireland, who were slaves for seven years upon arrival in the New World.
In the late 1800s, the term "White Trash" was used by wealthy sharecroppers known as Redeemers or Bourbon Democrats to describe white populists, who were trying to politically mobilize blacks and poor whites.
The myth that the Redeemers propagated was that all poor whites were violent racists, who were in the Ku Klux Klan. In truth, the first Grand Dragon of the KKK was a wealthy slaveholder named Nathan Bedford Forrest.
Today, the white trash stereotype usually gets misapplied to the white underclass in rural US. Generally a person gets the label, if that person chooses not to attend church, vote Republican or own enough land. Listening to heavy metal music or reading Stephen King books may compound the negative image, so that a person gets sent to prison (Google: West Memphis Three).
Historical white trash people: Harry S. Truman, Bill Clinton, Elvis Presley, Abraham Lincoln and U.S. Grant.
White trash people are just another racial stereotype like the wetback, nigger and wild Injun stereotypes.