What cardio is:
Cardio, short for cardiovascular fitness, is the
capacity of the heart and lungs to deliver oxygen to muscles to sustain a
movement over a long period of time.
A cardio exercise is any exercise
designed to develop and strengthen your cardovascular capacity.
Running
is a good example of a cardio exercise. When you move your legs back and
forth your heart and lungs have to supply oxygen to the muscles in your
legs. The muscles in your legs are so large that it takes a lot of oxygen to power them. Delivering this oxygen to your legs takes a lot of work from the heart, and this work will develop cardiovascular fitness.