Slavery is the ownership of a person as property, especially in regard to their labor. Slavery typically involves compulsory work, with the slave's location of work and residence dictated by the party that holds them in bondage. Enslavement is the placement of a person into slavery, and the person is called a slave or an enslaved person.
In the course of human history, slavery was a typical feature of civilization, and existed in most societies throughout history, but it is now mostly outlawed.
In the US, penal slavery was arguably illegal under a strict literalist reading of the Constitution until penal slavery was legalized in the aftermath of Civil War as a compromise.
In all countries of the world is outlawed at least this much (with tolerance of penal slavery being common) but in practice illegal slavery happens anyway. The Global Slavery Index (wikipedia link) has information on this.
Kant's third formulation of the categorical imperative lets you build up most of the structure of the key moral ideas from a simple rule: "treat no person as purely a means to an end, but always also as an end in themselves". Many applications of the categorical imperative require baroque derivations to loop back and be justified from this premise (treated as a generative axiom) but "consent ethics" in general, and "slavery is forbidden" are both elementary proofs from this starting point. A slave is a person, turned into a tool and piece of property of another person... a literal "means" to ANY end that the owning person (or "Master") deems desirable and feasible.
Historically, slaves would be kept in bondage for life, or for a fixed period of time after which they would be grated freedom. Many historical cases of enslavement occurred as a result of breaking the law, becoming indebted, suffering a military defeat, or exploitation for cheaper labor; other forms of slavery were instituted along demographic lines such as race or sex.
Although slavery is usually involuntary and involves coercion, there are also cases where people voluntarily enter into slavery (link to wikipedia!) to pay a debt or earn money due to poverty.
The issue has gained modern salience given the possible existence of "digital people" who are (1) owned as property, (2) perform cognitive labor, (3) aren't laboring consensually, and (4) don't get pay.