What is a Preposition?

Basically, a preposition is a part of speech.
Prepositions link a noun or pronoun with other words in a sentence.

E.G: The cat OF mine

Other function of prepositions is to introduce a prepositional phrase (PP) in a clause (in a phrase, acts as modifier).

The most common prepositions used in english are "of", "to", "with" and "for".