Grams per second of air iirc, through the MAF, you can see where it's located in the engine bay to the top left of the Engine Cover with wires going into it (very sensitive component) - in protrudes into the intake air after the Airfilter.
As Gaz says it's: (value MAF g / s) / 0.82 = hp That's what I used to plot the graph.
Using the other scanners, can you see what boost your're running? I think standard GTI is around 9-11 PSI, tuned should be nearer 20 iirc.
Sorry for being vague, it's been ages since I messed with it.