Slavery is an economic/legal status in which a person is seen as the property of another person. The slave is typically required to work for the person who owns them. Slavery can take many forms.
The thing that is common to all forms of slavery is that the slaves are not free. They are treated as property and therefore can be bought or sold. Because they are property, they do not have any of the rights that they should expect to have because of their status as human beings. The point of enslaving them is to require them to work for their owners.
Slavery was very common throughout much of history. We know, for example, that the Greeks and the Romans had slaves. Their slaves were typically people who had been captured in battle, but they also bought slaves from slave traders.
Beginning in the 1500s, slavery came to the Americas. Slavery in the New World soon came to be based on race. People were brought from Africa against their will and sold into slavery in the Americas. Slavery came to be associated with black people in a way that had not previously been the case.
Today, slavery still exists, though it is typically illegal. People are often coerced into slavery by others who take advantage of their poor economic circumstances.