This is a base Wordpress theme, which includes bill-bootstrap (a fully responsive HTML5 web framework).
The idea is that this can be used as a parent theme, for developers to create child themes from. The base theme includes all of the default functionality that a typical theme would need, including some extras, such as -
In theory, you could build a child theme without having to create any of your own theme files, just the style.css file, to determine your own colours, images etc. However, if you want to change the layout, it's just a simple case of creating your own version of any files that need to be changed.