CSS frameworks are pre-prepared libraries that help developers create websites more efficiently. They provide a set of standardized styles and components, such as buttons, grids, and forms, which can be easily customized. By using a framework, developers can save time and ensure consistency across their projects, making it easier to maintain and update their websites. Some popular CSS frameworks include Bootstrap, Foundation, and Bulma. These frameworks come with built-in responsive design features, allowing websites to look good on various devices, from smartphones to desktops. By leveraging these tools, developers can focus on creating unique content rather than starting from scratch with every project.