Well kiddo, back in the old days, when some grownups did bad things and got caught, they were sent to jail to pay for their bad actions. This was meant to help them learn from their mistakes and become better people.
But unfortunately, some people who were in charge of the jails, called wardens, didn't do a good job of taking care of the people who were sent there. Instead of helping them, they made them work really hard for little to no pay.
This is called convict leasing. It meant that these people who were already being punished for their bad behavior, were then forced to work for free or very little pay, and in very harsh conditions that were dangerous to their health. Sometimes, these people had to work in mines, factories or plantations, where they were treated terribly and their lives were in danger.
This practice was very wrong and thankfully it is not allowed nowadays. We believe that people who have made mistakes and have gone to jail deserve to be treated fairly and humanely, and not as slaves.