About
This is id Software's official GPL source release for Quake III Arena, including the full renderer and game code plus supporting tools — the bytecode compiler chain, a BSP map compiler, and the Q3Radiant level editor. Some individual components, including zlib-based archive handling, an MD4 implementation, a BSD-licensed libc replacement, ADPCM audio code, and the Independent JPEG Group's JPEG library, ship under their own separate, non-GPL licenses rather than the GPL.
Getting started
Building requires Visual C++ 2003-era project files on Windows, or gcc 2.95 with nasm and X11's Direct Graphics/Video Mode extension headers via a bundled build script on Linux. id's own notes admit the release wasn't extensively tested after upload and suggest looking for a community-maintained, more modern build of the codebase instead.
History
id Software notes that whenever one of their games' source is released under the GPL, community projects typically spring up to modernize and clean up the codebase for current compilers, and suggests seeking those out if this release is being picked up long after its original upload.