Stretching is as important as doing the training session for many reasons. It increases circulation and blood flow to the heart and muscles. It increases flexibility and range of motion and strength. It also reduces soreness of the muscles assisting in muscle recovery. Stretching decreases injury, stress on the joints and improves posture.
When stretching it should be in a static movement, which means taking the muscle to the point of resistance and holding the position for 20 to 30 seconds. To increase the stretch you can gently push the resistance a bit more in the same position. Please ensure that you do this carefully and cautiously.