A relative clause is a type of subordinate clause that provides additional information about a noun in the main clause. It usually starts with a relative pronoun (such as who, whom, whose, which, or that) or a relative adverb (such as when, where, or why). For example, in the sentence "The book that I bought yesterday is really interesting," the relative clause is "that I bought yesterday" and it provides more information about the noun "book."
