Bootstrap (https://getbootstrap.com) is a responsive UI framework for modern web development. It offers a mobile-first approach for web development with utilities and UI components that are fully responsive. Bootstrap 4 is the latest version, uses flexbox for layout, and is completely written in Sass, which makes it easier to customize. Bootstrap supports a 12-column grid framework, which lets you build responsive web pages with ease. JHipster uses ng-bootstrap (https://ng-bootstrap.github.io) so that pure Angular components are used instead of the ones provided by Bootstrap, which are built using JQuery, and Bootstrap is used only for styling.
Mobile first web development is an approach where the UX/UI is designed for smaller screen sizes first thus forcing you to focus on the most important data/elements to be presented. This design is then gradually enhanced for bigger screen sizes making the end result responsive and efficient.