Stretching is very important for flexibility, range of motion and injury prevention. Stretching also increases blood flow to the muscle.
There are 2 types of stretching:

• dynamic stretching
• static stretching

Dynamic stretching you do before you start your workout. The purpose of it is to prepare the muscles and your body – and also your mind – for the workout. It consists of putting your muscles through their full range of motion by way of mobilizing the joints they’re attached to. Dynamic stretches will elevate the muscles’ core temperature and ramp up the nervous system so that your body is ready to lift some heavy weights. It will help reduce the risk of injury and over time it improves your performance and maximizes your movements due to the increase of flexibility in your joints.
Static stretching you use during and after your workout. It helps cool down the muscles, prevents injury and makes sure you lengthen your muscles after contracting/shortening them during resistance training.