A prefix is a group of letters added to the beginning of a word to change its meaning. For example, the prefix un- means "not," so when added to the word happy, it creates the word unhappy, which means "not happy." Prefixes can help form new words and expand vocabulary. Prefixes are commonly used in the English language and can indicate various meanings, such as negation, repetition, or location. For instance, the prefix re- means "again," as seen in the word redo, which means "to do again." Understanding prefixes can enhance reading and comprehension skills.