Congress passed the amendment in early 1865, with the House vote on January 31. It was ratified in December 1865, eight months after the Civil War ended. Section 1 bans slavery "except as a punishment for crime whereof the party shall have been duly convicted." Section 2 gives Congress power to enforce it.
Southern states used the exception almost immediately. Black Codes made it a crime to be unemployed, to travel without a pass or to break a labor contract. People convicted under those laws were leased to plantations, mines and railroads. This system, convict leasing, lasted until Alabama ended it in 1928.
The exception still matters. Incarcerated people can be required to work for pennies an hour or nothing. Since 2018, voters in several states, starting with Colorado, have removed similar exceptions from their state constitutions. A proposed federal Abolition Amendment would remove it from the US Constitution.