It not a programming language it's a CSS framework. I use bootstrap all the time for quickly styling up prototype applications. It essentially allows you to create a good looking website without having to write any CSS.

You want to know what bootstrap is? Go look: Bootstrap

No one here is going to be able to explain to you what Boostrap is better than its own documentation (which is excellent, btw.) so go read it.

Bootstrap is not a programming language. It is a html, css, javascript framework that you can use it for creating a web sites or web applications.