Bootstrap is best known as a framework for building responsive websites with an intuitive grid system. You could essentially build a responsive website right out of the box if you wanted to as most every component you’d want to include (buttons, navigation menus, typography, sliders, cards, etcetera) comes pre-styled and ready to go, just as long as you have a handle on your HTML and CSS. Plus it’s well-tested for compatibility and consistency across browsers and is pretty robust under the hood. The heavy lifting has pretty much been done for you already.