The document defines a preposition as a word that governs and usually precedes a noun or pronoun to express the relationship between that noun or pronoun and another element in a clause. It provides examples of simple prepositions like "under", "behind", and "among" and explains they show the relationship between a subject and object. The document also defines compound prepositions as those formed by prefixing a preposition to a noun, adjective or adverb, and phrasal prepositions as a series of words or phrases that relate the object of a preposition to the rest of the sentence.