A relative clause is a subordinate clause that provides information about the antecedent in the main clause. The antecedent is the word in the main clause that the relative clause refers to, such as the actor or film. Relative pronouns like who, which, that, and whose are used to link the relative clause to the antecedent. Defining relative clauses add essential information about the antecedent using pronouns like who for people and which for things.