Prepositions are words that show the relationship between a noun or pronoun and another word in the sentence. They often indicate location, direction, time, or manner. While many prepositions are short, such as "on," "in," "at," and "to," they all consist of at least three letters.
Perhaps you were thinking of two-letter words that can sometimes act as prepositions, such as "up" and "on." These words can be used as prepositions in certain contexts, but they also have other grammatical functions.