Camping refers to the practice of a resident remaining in one location and not interacting with the world, (often leaving Second Life to run on their computer unattended). These avatars are often derogatorily referred to as "zombies."
Land owners would place "camping chairs" on their property, setting them up to pay out a certain amount of Linden Dollars for every "x" amount of time a resident spent on that land. By doing this, the owner raised their traffic score and, under the old system, was paid more by Linden Lab. Some residents even went to the practice of using programs that would automatically send a mouse click to the Second Life client to prevent the system from logging them out due to inactivity.
Linden Lab had strongly been against using the traffic system in this way and, as of June 13 2006, the Linden Lab payout system for high traffic areas was phased out altogether. Camping chairs still remain, though it is unclear for how long, since traffic is still used to denote "popularity" in the current Search system.