From Valve Developer Community
$ignorez (as in "ignore the Z axis") is a boolean setting that enables cheap sorting. It is mostly used for interface materials, which will always be in front of the 3D world, and skybox materials, which will always be behind.
Note:$ignorez can be used on any textures or models, which can be used to cheat by modifying the game files. This has never been fixed, as it would break certain models that depend on it.