Bootstrap will figure out how wide your screen is and respond by resizing your HTML elements - hence the name Responsive Design.

With responsive design, there is no need to design a mobile version of your website. It will look good on devices with screens of any width.

You can add Bootstrap to any app just by including it with

<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.1/css/bootstrap.min.css"/>

at the top of your HTML.