Bootstrap is a free collection of tools for building websites and web applications. It contains HTML and CSS-based design templates and components for things like navigation bars, dropdown menus, responsive image galleries. Developed by Twitter, Bootstrap is open source and one of the most popular projects on GitHub. It allows developers to build responsive, mobile-first sites or web applications quickly without having to write CSS code from scratch. Some key advantages of Bootstrap include being easy to use, having a large selection of customizable components and features, extensive documentation, and not requiring developers to write their own CSS.