A preposition in English grammar is a word employed with a noun or pronoun to form a phrase, a prepositional phrase. Prepositional phrases express meaning closely related to the meaning expressed by adverbs and adjectives: where, when, how, and what kind. Prepositional phrases can also express a sense of why or who.

"He's in the jailhouse now." (where)

The word in is a preposition. The word jailhouse is a noun. The noun is called the object of the preposition. This prepositional phrase provides a sense of location, a sense of where something is located. The quoted words are part of a song from the movie, Oh Brother, Where Art Thou?

I put your book in my locker. (where)

I am in my car, and I have a flat tire. (where)

Yesterday, I was at the beach. (where)

The road runs through a tunnel. (where)

Your pencil is on the floor under your desk. (where)

I shall be home by Friday. (when)

Meet me between classes. (when)

It was the beginning of summer. (when)

I will be there for a week. (duration of time)

He was arrested for jaywalking. (why)

I came with Camile. (who)

With perseverance she will become the best gymnast. (how)

I was with her when the announcement was made. (who)

I want a candy bar with peanuts. (what kind)

displaced prepositions

Which lot did you park the car in? (You parked the car in which lot?)

She is someone I could work with. (I could work with her.)

What does your dog look like? (Your dog looks like what?)

Whom would you like to be stranded on a tropical isle with?

Juanita is the girl I would like to be stranded with.

NOTE. Although we frequently use displaced prepositions in casual conversation, displaced prepositions are rarely used in carefully crafted documents.