Suffixes are groups of letters added to the end of words that change the meaning or function of the word. The document provides 26 common suffixes used in English and examples of words that use each suffix, such as "-er" to form words describing people like "writer", "-or" to form words describing people like "actor", and "-ness" to form words describing qualities like "happiness". The document aims to help the reader understand what suffixes are and provide examples of common suffixes in English.