When a prepositional phrase modifies a noun or a pronoun, it is called an adjectival prepositional phrase or an adjective phrase.

Read the paragraph. Notice the boldfaced words and italicized phrases.

The old house at the side of the hill stands all alone. An old dirt road at the bottom of the hill ran from the house to the countryside. There lay a field of clover and flowers between the road and the hill. The hill itself was bare and rocky. A thin cluster of trees below it looked like lonely guards. It seemed that the dark lonely house, in conflict with all living things, was really a mystery.

The italicized phrases modify or describe the boldfaced words before them. The italicized phrases are prepositional phrases. The words in boldface are nouns. The prepositional phrases are called adjectival prepositional phrases or adjective phrases because they modify a noun or a pronoun.