V02 max is a measure of the maximum volume of oxygen that an one can use.
As you increase your effort when you exercise, the amount of oxygen you consume to produce energy increases. However, there is a maximum level of oxygen consumption, beyond which increases in exercise intensity don’t lead to further increases in oxygen consumption. This level of oxygen consumption is called the VO2 max.
There are loads of calculators out there- but I'm just not that technical :(