A prepositional phrase is a group of words which begin with a preposition and end with a noun or noun equivalent which acts as its object. That noun equivalent could be a pronoun, a gerund, a noun phrase or a noun clause.
Here are some examples of the most basic prepositional phrases: at home, in time, on time, with me, by foot, in line etc.
The noun or pronoun which acts as the object of the preposition could be modified by an article, demonstrative or some other determiner.
In the corner
Here the article the modifies the object corner.
It is a gift from my great grandmother. (Prepositional phrase – from my great grandmother.)
We need to have a discussion about the consequences of this move. (Prepositional phrase – about the consequences of this move)
The cat was sleeping under the warm blanket. (Prepositional phrase – under the warm blanket)
Role of a prepositional phrase
A prepositional phrase usually acts as an adjective or adverb. When used as an adjective, it answers the question which one?
The pretty girl in my class has stopped talking to me.
Which pretty girl? The one in my class
The library across the street is well-stocked with books.
Which library? The one across the street
The girl from Mexico impressed everyone with her quick wits.
Which girl? The one from Mexico.
When used as an adverb, the prepositional phrase will answer the question How? When? or Where?
I took a nap in the afternoon.
When did I take a nap? In the afternoon