What is the function of aperpositon

I assume you meant to type "preposition" which is a word that describes the relative location or POSITION of a noun. Examples include about, during, after, along, within, to, near, and so forth. Hope this helped.

If you mean to say preposition, is a word that connects nouns, pronouns, and phases to other words. They often precede a noun or pronoun. Examples are "He walked in the room." or "He walked through the door."