A preposition is used to link noun, 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 is used to indicate the temporal, spatial or logical relationship of its object to the rest of the sentence. Here are some examples:
The pencil is ON the desk.
The pencil is BENEATH the desk.
The pencil is leaning AGAINST the desk.
The pencil is on the floor BESIDE the desk.
He held the pencil OVER the desk.
He wrote with the pencil DURING class.