A morpheme is the smallest unit of meaning in a language. It can be a word itself, like cat, or a part of a word, such as the prefix un- in unhappy. Morphemes can be classified as free morphemes, which can stand alone, or bound morphemes, which must attach to other morphemes to convey meaning. Morphemes play a crucial role in understanding how words are formed and how their meanings change. For example, adding the suffix -ed to the verb walk creates the past tense walked. By studying morphemes, linguists can analyze the structure and evolution of languages.