A prepositional phrase (/prepəzɪʃənəl freɪz/) is a phrase with a preposition as its head. They consist of a preposition and a prepositional complement.
Prepositional phrases express various aspects of meaning, including agency, place, etc.
Prepositional phrases usually (always?) function as adverbials or adjectivals, but can also function as nominals.
The form of prepositional phrases varies somewhat. A preposition may license various prepositional complements.