Adjective clauses function as adjectives. They describe nouns and pronouns. Most adjective clauses start with the pronouns who, whom, which, that, whose, when, or where. Other pronouns like whoever, whatever, whomever, whichever, what and why can also start an adjective clause. Adjective clauses that begin with one of the relative pronouns are also called relative clauses.
We set out for the next town where we had planned to stay the night. (The adjective clause where we had planned to stay the night describes the noun town.)
The boy who stole the watch was caught. (The adjective clause who stole the watch describes the noun boy.)
This is a fine opportunity which should not be lost. (The adjective clause which should not be lost describes the noun opportunity.)
I found a quiet place where we can meet. (The adjective clause where we can meet describes the noun place.)
Here is the book that you were looking for. (The adjective clause that you were looking for describes the noun book.)
Have you ever spoken to the people who live next door? (The adjective clause who live next door describes the noun people.)