Bootstrap is a popular, open-source CSS framework for developing responsive and mobile-first websites. It provides a comprehensive collection of pre-built components, including navigation bars, forms, buttons, and grids, simplifying front-end development. Bootstrap's responsive grid system adapts seamlessly to various screen sizes, ensuring optimal viewing experiences on desktops, tablets, and smartphones. It offers extensive customization options through Sass variables, allowing developers to tailor the framework to their specific design requirements.