VO2 max is a measure of the maximum volume of oxygen that an athlete can use. It is measured in millilitres per kilogramme of body weight per minute (ml/kg/min).
As you increase your effort when you exercise, the amount of oxygen you consume to produce energy (and hence the rate at which you exhale carbon dioxide) 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. (The initials simply stand for volume of oxygen. )