When you are participating in physical activity and or exercise it is important to make sure you warm up, cool down and stretch.
Warming up prepares the body for more strenuous work, or exercise. It takes a few minutes for the body to gear up to work optimally during an exercise bout. Firstly blood has to be redistributed from your internal organs to your working muscle, your heart and lungs. The movement of blood to your muscle warms them which reduces your risk of injury as your move your body more strenuously. Therefore a warm up is important to prepare your body for exercise and reduce your risk of injury.
Similarly a cool down in important because as you exercise your heart rate is increased, and your body temperature is higher. A gradual cool down is an effective way of reducing post exercise muscle soreness.
Stretching is an important part of a cool down down for many reasons. Stretching is important in maintaing range of motion around a joint a.k.a. flexibility. It also reduces muscular tension built up in the muscle during exercise, thus improving recovery time. Stretching must be done while your muscle are still warm to prevent injuring yourself, and is a great way to cool down; you are still moving, but you are giving your body a chance to recover gradually.