From Quake Wiki
QuakeC is a language developed by id Software for their smash hit game, Quake. It is inspired by the C programming language, but has many many differences. While not particularly powerful, it made Quake one of the most easy to edit games of its day and spawned literally millions of mods. In fact the total quantity of mods easily exceeds Quake 2, Quake 3, and Half-Life combined. However, most of them are no longer available as the great hub of the community, PlanetQuake lost interest in Quake, when Quake 2 came out, and started dumping sites and mods left and right. Another major hit to the community is that cdrom.com stopped, a long time ago, accepting uploads for Quake and even the new location of the community archives doesn't accept uploads.
But anyway, QuakeC is compiled with a QCC program (such as FrikQCC or FTEQCC) into a progs.dat file. The file is read by the virtual machine inside the engine. This method is incredibly secure and cross platform, much like Java.