I believe in u/pizlonator's parlance Yolo-C is any C compiler that isn't Fil-C.

Some parts of Pizlix are built with Yolo-C, including the kernel. My guess is that Fil-C is not yet able to build a Linux kernel and have it run well, or maybe it's only a bootstrapping issue (as u/pizlonator notes in another sub-thread).

Fil-C _itself_ needs two C libraries: one for Fil-C's run-time, and one for the program you've compiled with Fil-C. The C library for the program built with Fil-C has to be built with Fil-C, while the one for Fil-C's run-time can't itself be compiled with Fil-C and must be built with Yolo-C.

Lastly, YOLO is an acronym meaning "you only live once". It's something one says right before doing something dangerous, such as bungee jumping. The joke here is that any C compiler that is not the Fil-C C compiler is dangerous, thus "YOLO!".