Read the following sentences.

1. That boy across the room is my best friend.

2. The boy ran across the room to meet his friend.

In the first sentence, the word boy is modified or described by the prepositional phrase across the room. Boy is a noun. Across the room is used to modify the noun boy; thus it is an adjectival prepositional phrase.

In the second sentence, the word ran is described by the prepositional phrase across the room. Ran is a verb. Across the room is used to modify the verb ran; thus it is an adverbial prepositional phrase.