I'm not proficient in C. Does this "just work" or do you need to provide some sort of rendering environment like SDL?
Yes, the repo offers a couple of example renderers.
The idea is that you already have a rendering pipeline (e.g. in your game engine), but want to lay out more complex UIs in it. Then you can use this library to make nice settings / chat / stats / whatever screens, or even render realistic content onto screens of in-game computers, pages of books, etc.