In grammar, a preposition is a part of speech that introuces a prepositional phrase. For example, in the sentence "The cat sleeps on the sofa", the word "on" is a preposition, introducing the prepositional phrase "on the sofa". In English, the most used prepositions are "of", "to", "in", "for", and "on". Simply put, a preposition indicates a relation between things mentioned in a sentence.