Despite that poster being quite vague, I believe what they're referring to is the existence of Peonage well into the twentieth century. A form of slavery known as Debt Slavery in which a person owed money to another and would work off the debt with labor. This form of slavery was legally outlawed by Congress in 1867 but lasted well into the 1940s-1960s through legal trickery and lax dispensation of the law.

The best book on it is Pete Daniels The Shadow of Slavery: Peonage in the South, 1901-1969