A root word is the basic part of a word that carries its primary meaning. It is the foundation upon which other words are built by adding prefixes or suffixes. For example, in the word unhappiness, the root word is happy, which conveys the core idea of joy or contentment. Root words are essential in understanding the meanings of related words. By recognizing the root, one can often deduce the meanings of unfamiliar words. For instance, the root word bio means life, appearing in words like biology and biography, which relate to the study of life and the story of a person's life, respectively.