Meaning "the business", de_Biz refers to the Video Game Industry. It could also refer to Valve Corporation.
The term is a reference to the naming system of maps in games created by Valve, particularly Counter-Strike. For example, de_Dust has the prefix "de_" meaning "DEathmatch" and denotes that the map is designed for the deathmatch gametype.
Valve Corporation is my favorite company in de_Biz. Their games just pwn so much!