Also called prerendering sometimes.
Rendering a page (mostly on the developers machine or a service for it) to static HTML instead of doing this for each user request on the app’s server (Server-side-rendering).
What I love about SSG / prerendering is, that you can distribute your app easily on a CDN (or anywhere else), as your app can run without a server rendering, it is therefore quite runtime-agnostic.
You can build full-stack apps nontheless, featuring Authorization, DB calls, etc. with using HTTP APIs to call from the client
Keep in mind, this is also possible with classic Client-side-rendering, as you just ship a bundle of JS + HTML and CSS from a “static” web server.