Affixes are groups of letters added to the beginning or end of a word to change its meaning. There are two main types: prefixes, which come before the root word, and suffixes, which are added at the end. For example, in the word unhappy, the prefix un- changes the meaning of the root word happy to its opposite. Affixes help create new words and expand vocabulary. By adding different affixes, one can modify a word's tense, number, or meaning. For instance, adding the suffix -ed to the verb play forms the past tense played. This process allows for greater expression in language.