Prepositions are words like in, during, or between. They are normally followed by a noun or pronoun, with which they form a prepositional phrase:
Here are some more examples:
between you and me
The job of the preposition is to relate the noun or the pronoun (the object of the preposition) to the rest of the sentence. Take, for instance, the following sentence:
I work at the post-office.
The prepositional phrase tells us a bit more about where I work.