Cores*Clocks*2 (FLOPS)/1 Trillion. So, for example, the Titan Xp has 12.15 of FP32 (float) performance, right? That's 3840 Cores*1582 [1,582,000,000] (Boost Clock)*2 FLOPS per clock cycle /1,000,000,000,000 = 12.14976 (12.15).
It's as simple as that (though hardly something people should use when talking about actual performance, but it has become a major buzzword in console gaming of late). Makes sense in that instance because it shows you how much more improved a machine is versus the previous, but cannot give you a full scope of the bigger picture.