If you don't have time to watch the documentary, just read the article. It only takes 5 minutes, and you'll probably learn something about American history.
TL;DR: Basically, while widespread slavery ended after the Civil War, what replaced it for an estimated 150,000 blacks (rather than the 4,000,000 blacks at the start of the Civil War) was punishment for crimes such as "vagrancy".
You could be sentenced to years of hard labor for a variety of offenses, and you would then be "rented" to landowners or to companies such as U.S. Steel.
This all ended at the outbreak of WWII when FDR told all the US Attorneys to strictly enforce existing Federal laws against “involuntary servitude and slavery”.
I'm stunned. I know that there was still tons of institutional racism after the Civil War, and I know that racism is still rampant, and there's still a long road to go there, but I was honestly under the impression that actual slavery at least ended with the Civil War.
http://www.pbs.org/tpt/slavery-by-another-name/watch/