The warning comes when humidity levels are very low and wind predictions are very high. Even the tiniest spark can start a fire that grows explosively into a blowup. (Oh yeah, I wrote a book about that.)
So no yard work (a shovel hitting a rock can cause a spark), no campfires or barbecues (a hot ash can drift into vegetation), no parking off paved roads (a vehicle's exhaust and catalytic converter can ignite vegetation) and no cigarettes smoked outdoors.
Please stay safe.