Adjective clauses do what adjectives do; they modify nouns. Adjective clauses usually consist of a pronoun (such as who, whom, that, which, or adverbs such as when, where, or why) and these clauses includes a subject and a verb.
For example, this first sentence is without an adjective clause and the second contains the clause. The clause doesn't usually change the meaning of the sentence; it just adds information.
Structured buildings last a long time.
Structured buildings that are designed by architects last a long time.
The phrase "that are designed by architects" modifies "buildings" in the same way an adjective (like "structured") does.
Presidential candidates should not lie.
Presidential candidates who are running for office should not lie. (adjective clause)
Presidential candidates running for office should not lie. (adjective phrase)
An adjective clause can be condensed to an adjective phrase by deleting the pronoun or adverb and the verb ("who" and "are").