It's the total time that you spent in the air by doing jumps/flying off ramps. Eg. Turbo boosting for the shortcuts on Sebenco + Ubermall give you lots of airtime.

But it should only be very small... you spent more time in the air than you've played wipeout. It's probably bugged... like the 'Photos Taken' for some people is in the tens of thousands.

I have a game time of 154:25:08 and a 'time spent in the air' of 14:05:09.