An adjective clause is a dependent clause that, just as an adjective, modifies a noun or a pronoun. It begins with words such as when, which, who, whom, whose, why, that, and where.
Adjective clauses can be either essential or nonessential. An essential adjective clause, also known as restrictive adjective clause, provides information that is necessary to identify the word it modifies. On the contrary, nonessential adjective clauses provide additional information about the word it modifies. Nonessential adjective clauses are set off with commas, while the essential ones are not.