They refer to what used to be called the Ranger Unit (they use some other term now). Generally, the first two letters indicate the county or counties covered by the unit. Because of consolidations, some units cover significantly more territory than their name implies - example: LNU is the Lake-Napa-Sonoma unit (formed by a merger of the Lake-Napa and Sonoma units) covers all or part of 6 or 7 different counties.
Go to http://fire.norcalscan.org/ and get the "CA Quick Fire Guide" for a complete list, with rough boundaries.