VO₂max

VO₂max is the maximal velocity of oxygen uptake. Or in layman terms the maximum rate at which an organism (you) can process oxygen.
The general principle being that the higher the rate, the fitter / better athlete you are. Despite this, it is actually a poor indicator of performance, e.g. race completion time.
This has not stopped well known sporting manufacturers from using it for such purposes – nice for marketing, but not actually any use.