Does that mean, you go on a waiting list at the airport and how does it work?

I'm looking at flights to from SJC/SFO to DCA or surrounding. I see that most of the flight from the West to the east coast doesn't have the instant upgrade. But the flight form DCA to LAX does. The ticket is almost $1050 vs $$650 between the best deal vs value fares and the Full Flex is $1300.