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. 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, play is a free morpheme, whereas ed in played is a bound morpheme that indicates past tense.