I am by no means an expert in this, and someone else may have better, clearer information, but I understand VO2 Max (where O2 stands for oxygen) to be the maximum amount of oxygen delivered to your muscles while exercising. The higher the number, the more fit you are and the more calories you burn while exercising.
Heart Rate monitors estimate this number for you through a formula using age, weight, activity level, etc., to calculate how many calories you burn. Mine also has a "test" you can do to get a better estimate (lie down for three minutes and then activate the test, which counts a certain number of heartbeats, then gives you an estimate based on how long it took to get that many beats...confusing, I know).
The only accurate way to find out your VO2 max is a laboratory test on a treadmill, much like anareobic threshold (heart rate at which your body stops using fat for fuel). Estimates can certainly be helpful in detecting a change, though, as well as giving you a close idea of where you are. And I think it takes six weeks to effect a change.
Was that sufficiently complicated enough to confuse you more?
Kelly