Check out the videos from Jamstack Conf Virtual, May 27. Watch the presentations
Basically it's a Markdown to HTML processor, pre-packaged with BootStrap Bootswatches. And some extra tricks :
Unlike most static site systems that are built around templating engines, with BootDown you write both your page content, and the structure in a single source file. Using [. .] for divs. This gives you all the flexibility you need to layout your pages any way you like, within the BootStrap grid.