An antecedent is a thing that comes before something else. You might think rap music has no historical antecedent, but earlier forms of African-American spoken verse go back for centuries.
In logic, mathematics, and grammar, the word antecedent (from Latin ante-, "before" + cedere, "to yield") has the meaning "the first part of a statement." More generally, it means "something that came before, and perhaps caused, something else." The word is also an adjective: a lawyer or judge might talk about the "antecedent events" leading up to someone committing a crime.