Short answers, pulled from the story.
Slavery is the ownership of a person as property, especially in regard to their labour. It typically involves compulsory work, with the slave's location of work and residence dictated by the party holding them in bondage. It is fundamentally an economic phenomenon.
In 2019, approximately 40 million people were still enslaved throughout the world, of whom 26 percent were children. More than half of today's slaves provide forced labour, usually in the factories and sweatshops of the private sector.
Mauritania was the last country in the world to officially ban slavery, doing so in 1981. Legal mechanisms to prosecute slaveholders there were not implemented until 2007.
In chattel slavery, the slave is legally rendered the personal property of the owner and can be bought and sold like livestock. Debt bondage, or indenture, is unfree labour in which a person works to pay off a debt by pledging themselves as collateral, and it can pass from one generation to the next.
Writing in Le Monde diplomatique, Elikia M'bokolo estimated eleven to twenty million people taken across the Atlantic, around nine million along the trans-Saharan route, and roughly four million each via the Red Sea and the Swahili ports of the Indian Ocean. Brazil alone received nearly 5 million African slaves between 1501 and 1866.
The 13th Amendment, ratified in 1865, banned slavery and involuntary servitude except as punishment for a crime whereof the party shall have been duly convicted. This clause provided the legal basis for convict leasing and the prison labour that continues today, with the Prison Policy Initiative citing a 2020 US prison population of 2.3 million.