Bootstrap is a front-end framework for building responsive, mobile-first websites. It provides HTML and CSS-based design templates for common user interface components like buttons, navigation bars, and grid systems as well as extensions for things like carousels and modals. Developers can include Bootstrap via a content delivery network or by downloading files and linking the CSS and JavaScript in their HTML. It is a popular and widely used framework that helps speed up the development process and ensures a consistent and mobile-optimized experience across browsers and devices.