What is "bootstrap-3"

Bootstrap is a free collection of tools for creating websites and web applications. It contains HTML and CSS-based design templates for typography, shapes, buttons, navigation and other interface components, as well as optional Javascript extensions.

Bootstrap is compatible with the latest versions of all major browsers. It loses quality gracefully when used in older browsers such as Internet Explorer 8.

Since version 2.0 also supports responsive web design. This means that the layout of the web pages adjusts dynamically, taking into account the characteristics of the device used (desktop, tablet, mobile).

Starting with version 3.0, Bootstrap approved a first design philosophy for mobile, with an emphasis on responsive design by default.

It’s open source and available on Github. Developers are encouraged to participate in the project and make their own contributions to the platform.

Source: Wikipedia.