Its essentially a measure of your maximal oxygen consumption, the higher the better. Last time i measured mine i got around 47. The most accurate way to measure is being hooked up to a bike with a mask thing on, otherwise im pretty sure there are graphs that can be used to estimate with vo2 on the y axis and heart rate on the x axis..i think.
Well fitness correlates positively with vo2, the fitter you are the higher your vo2. Theres a lot of things you can do to improve that. 47's around the moderate area, which is quite good considering i dont play any sport anymore except surfing.