its a server variable to force your camera to different settings on death,
they added a new default value of 3 for tf2 - which allows spectators to spec everything, but for players in game on red/blu teams they can only spec their own team and via chase/3rd person views,
0,1,2 are all various different options of this, 1 used to be the preffered setting in tf2 (certainly i had it in my server.cfg) and this value is crashing servers left right and centre currently, if the amount of messages on hlds mailing list are anything to go by
if you are just running a listen server on your home pc it will default to 3.