A morpheme is the smallest unit of meaning in a language. It can be a whole word, like cat, or a part of a word, such as the prefix un- in unhappy. Morphemes help us understand how words are formed and how their meanings can change when different morphemes are added. Morphemes are classified into two main types: free morphemes and bound morphemes. Free morphemes can stand alone as words, while bound morphemes must attach to other morphemes to convey meaning. For example, in the word dogs, dog is a free morpheme, and s is a bound morpheme that indicates plural form.