What is "debt peonage" as noted in the nonfiction book, The Warmth of Other Suns?

World War II caused a serious labor shortage in America, and as a result, authorities in Florida arrested black men who appeared to be idle or unemployed. These men were charged with vagrancy and forced to work as pickers in the citrus fields. The money they earned was then handed over to the state to cover their “fines” for violating the law. These “captured” workers were beaten or shot if they tried to escape. This practice, known as debt peonage, was considered a form of slavery and was, of course, illegal.