A preposition is a word that shows the relationship of one thing to another. Prepositions tell us about position.
Example: Martin fell down the stairs.
Example: The ladies sat on chairs.
Example: I will sing a song for you.
Note: Some words can be used as either preposition or adverb.
Example: Sam and I left Dad inside the house. [A preposition is used.]
Example: Sam and I left Dad inside. [An adverb is used.]