To enslave someone is to force that person to work for no pay, to obey commands, and to lose his or her freedom. The ancient Greeks were known to enslave groups of people they defeated in military battles.
It's less common today for one group of people to enslave another, but unfortunately it does still happen. You might also encounter enslave being used in a figurative way, to describe something that takes over a person's freedom to make choices in some way: "Being hooked on cigarettes just enslaves her — she tries to quit, but she can't." Enslave comes from the "make into" prefix, en-, and slave, "person who is the property of another."