Cooling down, as in, cooling down after a workout. It doesn't seem too important, does it?
Cooling down is actually important. What is 'cooling down' defined by? A cool down is a period of time for example, at the end of a workout session, when you gradually decrease the physical activity of your body to almost resting. It usually consists aerobic moves, together with stretches.
It is important for several reasons, one of them is that it will help remove the metabolic waste your body might have acquired during the workout.
There are many more reasons though, for example if you feel faint after exercising - you do not cool down properly, or at all. Why would you feel faint? During physical activity, say, a workout, blood is pumped around your body by your heart. It circulates. The blood comes in to the heart, gets pumped out, and returns, repeat. During exercise blood is assisted in returning to the heart by muscle contractions and several other factors. If you suddenly stop all physical activity - your heart will continue to pump the blood around your body, but nothing will assist the blood in returning. And that is the reason why you feel faint.
If you properly cool down, that is, gradually decrease physical activity to almost none you heart will slow down with the slow decrease in the intensity, preventing you feeling faint.
You don't have to do stretches or aerobic movies to cool down, if you're jogging, you can begin decreasing the speed and intensity into walking, and from walking into slowly stopping for at least half a minute. Stretching after a work out though, is recommended.
The length of a cool down varies from person and exercise. If the exercise was strenuous - obviously your heart has been working intensively and will need a longer gradual cool down. If you are out of shape, or just a beginner you might also have to take it easy, as your body and heart might not have adapted to this kind of stress yet. Along with warming up, cooling down, as you see, is important!