It's basically jsfiddle or codepen for nodejs. I have to admit, it's pretty interesting. I'm bias as I look at this from a teaching perspective and it makes it a lot simpler to just get into the meat. Looking at the code, there's a bit of magic. If you spin up the parse server it states that it requires a monogodb ENV key yet it'll just work without one. I'd imagine they are spinning up services behind the scenes.

Doesn't look like there's currently any way to add more modules. At least I haven't found it yet. Interested in testing out the import feature...

You could write it off as 'just' a web-based Node JS IDE. That is a large part of what it technically delivers. But here's a few points that we think makes it more useful and accessible to a much larger group of users than would ever typically use such a thing: https://gomix.com/about/