CSS preprocessors are scripting languages that extend the capabilities of standard CSS. They allow developers to write styles in a more dynamic and efficient way by introducing features like variables, nesting, and mixins. Popular preprocessors include Sass, LESS, and Stylus, which help streamline the styling process and improve code organization. By using a preprocessor, developers can create reusable code snippets and maintain consistency across stylesheets. This leads to easier maintenance and faster development, as changes can be made in one place and reflected throughout the entire project. Ultimately, CSS preprocessors enhance the workflow of web design and development.