Prepositions

A preposition is a word in a sentence that tells us about position of a noun or pronoun.

Examples :
Hold the ball in your hand.
Put the ball on your desk.
Put the ball under the desk.
Hold the ball above your head.
Put the ball behind the desk.
Keep the ball in front of the desk.

Your friend changed the place of the ball each time you told him to do an action.

Preposition words are also called position words.