We know that adjective is a word which qualifies noun.                                                                           A phrase which is used as an adjective is called as Adjective Phrase. That is, this phrase does the work of an adjective.

Examples -
a. The girl wore a dress of cotton.
b. I purchased a bicycle with a red basket.
In sentence ‘a’, the phrase ‘of cotton’ describes the noun ‘dress’. So ‘of cotton’ is an adjective phrase. It is made of ‘of’ (preposition) and ‘cotton’ (noun).