Believe it means they have passed the new emissions standards for trucks. They get a little badge to make the truck feel special but it in no way means a crock of do-do as far as idling or anything else.
It should mean that it is legal to idle that truck in California, because not all new engines have the clean air decal. Unless they changed this again. The sticker is an easy identifier for "law enforcement" personel that that engine is a new engine. I remember hearing there is an extra cost for "clean air certified".
