What is an adjective clause?

It is a dependent clause used as an adjective within a sentence.

An adjective clause is a dependent clause that, like an adjective, modifies a noun or pronoun. Adjective clausesbegin with words such as that, when, where, who, whom, whose, which, and why. An essential (or restrictive) adjective clause provides information that is necessary for identifying the word it modifies. it tells when where why