VO2 max is basically a test to determine your maximum possible energy output. Its a measure of endurance athletic capability, and they'll measure it either on a bike or treadmill depending on which you prefer and generally use the most. When your muscle burns a glucose, it'll consume O2 and produce CO2; they measure the maximum rate that your body can do that.
You've probably seen on TV occasionally some athlete running in a gym like atmosphere with a face/oxygen mask type get up on? That's what they're doing. Its very... not fun, but the number, is very useful.
In the end, its just a number that goes in a weird, overly complicated looking formula; which you can then use to go accurately from measured heart rate, to calories burned over time.
There are meters, in the $200-$400 range that will accurately record your heart rate over time, allowing you to know total calories burned accurately even though your HR is not held constant as in normal real world exercise. They consist of a small transmitter/sensor that straps to your chest, and gives you hives, blisters, and abrasions for fun proof that you're super serious; and a small computer that often has a USB connector to attach to your PC. From that you can look like you're testing a space astronaut in the process of finding out how much you burned on your morning run.
Fortunately V02 max doesn't change very rapidly, so you can use it for quite a while as long as you haven't gone from running 3 miles in 26 minutes to running 3 miles in 21 minutes. (my fastest, I'm a slug)
All that said, you don't REALLY need to know that number, or be so complicated, but it is the only way to really prove it to yourself if you're having trouble believing the estimate of the high end heart rate monitors. Honestly though, the monitor's basic formula that makes internal estimates of all this stuff is plenty accurate for almost everyone's purpose; it'll ask your genetic sex, age, height, and weight, and that's all you generally need.
I wonder, given the audience of the forum, does a transman on T use F or M for the input. I know I lose substantial power and endurance as soon as the T is suppressed in my own body.. No idea how to answer that question.
nbedit: I hope I was nicer this time. Thinking pink fluffy thoughts....