The document defines a preposition as a word used with a noun or pronoun to show how that noun or pronoun relates to something else. It provides examples of prepositions like "in", "on", and "with". The document notes there are three kinds of prepositions: simple prepositions like "in" and "on"; compound prepositions like "about" and "between"; and phrase prepositions like "according to" and "in order to". It concludes with the student thanking the reader.