The metabolic cart is a gas analyzer.  It can calculate the volume of air inhaled and the volume of CO2 exhaled.  This allows the researcher to identify the maximal volume of air inhaled at a given workload (VO2max), which has a direct correlation with cardiorespiratory fitness.  The higher the VO2max the better fit the individual is, regarding cardiovascular fitness.  It can also tell the researcher how many calories a subject burns at rest (Resting Metabolic Rate).  It can also analyze VO2/VCO2 (volume of oxygen inhaled divided by volume of carbon dioxide exhaled or more commonly known as the RER Respiratory Exchange Ratio).  The RER tells the researcher what substrate is being burned, by the given subject, at a given workload (like Fats, Carbohydrates).