what are adjective clauses ?

The types of dependent clauses i.e. noun clause, adjectival clause and adverbial clause serve the purpose of a noun, adjective and adverb respectively.

An adjectival clause performs the same function as an adjective does.

For example: Reena, who arrived here yesterday, will do the work.
The chain, which is made of gold, has been stolen.

An adjective clause usually starts with a relative pronoun(who, which, whose, that) or a relative adverb(when, where, why). It modifies a noun or a pronoun. It has a subject and a verb.