Bootstrap is an open-source CSS framework that simplifies the creation of responsive websites and applications by providing a collection of ready-to-use styles, components, and scripts. Developed by Twitter, Bootstrap offers a flexible grid, buttons, forms, modals, carousels, and other standardized UI elements, allowing developers to quickly design modern and consistent interfaces. With its mobile-first approach, Bootstrap ensures smooth compatibility across various devices and screen sizes, while also allowing easy customization through variables and themes. This streamlines the design process while ensuring a professional and uniform appearance across all platforms.