A preposition links nouns, pronouns and phrases to other words in a sentence. The word or phrase that the preposition introduces is called the object of the preposition.
A preposition usually indicates the temporal, spatial or logical relationship of its object to the rest of the sentence as in the following examples:
The book is on the table.
The book is beneath the table.
The book is leaning against the table.
The book is beside the table.
She held the book over the table.
She read the book during class.