VO2 max (maximal oxygen uptake) is the maximum rate at which your body can absorb and use oxygen during intense exercise. It is usually reported in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min). A higher VO2 max generally means your heart, lungs, and muscles work together more efficiently to deliver and use oxygen, which is why it is widely used as a marker of cardiorespiratory fitness. Lab graded exercise tests with a breathing mask are the gold standard for measuring it; many wearables also estimate VO2 max from heart-rate data.
Source: Cleveland Clinic — What is VO2 max and how to calculate it