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. For example, adding the suffix s to cat creates the plural form cats, indicating more than one. Morphemes are essential in linguistics because they reveal how languages work. By studying morphemes, we can learn about the structure of words and how they combine to convey different ideas. This understanding is crucial for language development and communication.