Bootstrap is a popular front-end development framework that provides developers with a collection of pre-written HTML, CSS, and JavaScript code. It is used to create responsive and mobile-first websites and web applications, meaning that they can adapt to different screen sizes and devices. Bootstrap includes a wide range of customizable components and styles, such as forms, buttons, navigation menus, and typography. It also offers a powerful grid system that allows developers to create complex layouts easily. Bootstrap is maintained by Twitter and is available as an open-source project, meaning that it is free to use and can be customized to suit the needs of individual projects.