The document discusses prepositions, which are words that indicate a relationship between nouns or pronouns in a sentence. Prepositions show relationships of time, space, direction, duration or logic. Common prepositions include "about", "above", "across", and "after". Prepositions function to link a noun as the object of the preposition to another word. Examples are provided such as "The book is on the table" and "She read the book during class".