Warm-Ups and Stretching
It is important to gradually get your body ready to exercise by doing a warm-up. This is generally a slower and less intense version of the exercise(s) you are about to do. The name comes from a feeling of getting "warm".
There is a great deal of debate around stretching before or after you exercise. Generally, stretching after exercise is accepted as best practice. Stretching before you exercise can be beneficial if what you are about to do involves a great range of motion (moving a body part to its limits), such as kicking at items above your head.
Stretching beforehand is not beneficial when what you are about to do requires one muscle to keep another muscle or bone structure "in place" during an activity (i.e. heavy weight-lifting).