Post author here: As others said, it's a CMS for static site generators.

Traditional CMS's are opinionated server side applications that live outside of the site developer's workflow, and are subject to maintenance and a high potential for getting hacked. The advantage is eschewing all of that for a hackable React app that literally lives on a page on your site, and gaining the speed and security benefits of static development.

To better understand the methodology, check out https://JAMstack.org.