Cardiorespiratory fitness is a measure of how well your body is able to transport oxygen to your muscles during prolonged exercise, and also of how well your muscles are able to absorb and use the oxygen, once it has been delivered, to generate adenosine triphosphate (ATP) energy via cellular respiration (cellular respiration is a chemical process in your body's cells that converts the energy stored in the food you eat into the ATP form of energy that is recruited for use by your muscles). Essentially, your cardiorespiratory fitness level is a measure of the strength of your aerobic energy system. If you haven't already read the Exercise Energy Systems article you can do so to get a better understanding of what ATP is, what cellular respiration is, and what the aerobic energy system is (in addition to your body's other two energy systems).
