Essentially, it's a reverse proxy that serves a cached, pre-rendered HTML version of your site to bot traffic while sending real users onward to your actual website. It works kind of like Cloudflare (which it's built on top of).
You can also hit it with an API from your end if you want to just forward bots or aren't comfortable sticking somebody else's service in front of your site.
I really would like not to have node running on my server in order to get SSR! Which effectively means I'll either have to serve entirely static pages and then hydrate them based on some model client side (which isn't server side rendering by any definition of the term), or I have to recreate the entirety of the HTML rendering functionality for my server side in my language of choice. I guess there might not be a way around the second option?
One thing I'd love to be able to do is to do the classic thing of rendering things mostly as html templates on server side, but serving up JS for particular page components. I also don't want to use node for SSR if at all possible, but seems to be the only route available.
That way I can easily support non-js users of the app (bing struggles to index my site at all). It used to be a lot easier to do this, but I guess it's out of vogue now!