A suffix is a group of letters added to the end of a word to change its meaning or grammatical function. For example, adding the suffix -ing to the verb run creates the word running, indicating an ongoing action. Suffixes can also change a word's part of speech, such as adding -ly to the adjective quick to form the adverb quickly. Suffixes are important in the English language as they help to expand vocabulary and clarify meaning. They can indicate tense, number, or comparison, among other functions. Understanding suffixes can enhance reading and writing skills by allowing for more precise expression.