With a static stretch you hold a relaxed muscle under tension. This helps lengthen the muscle, which will have contracted after weight training, and provides several benefits. First, it will help with flexibility, so you'll be able to work your muscles across a wider range of motion, leading to bigger muscle gains. But stretching also helps reduce injuries as your muscles and tendons are less likely to tear when they are relaxed.
Stretching also improves blood flow to your muscles, helping to flush out toxins, meaning you'll be ready for your next workout sooner. And stretching can also aid posture, because tense muscles can pull your spine, shoulders and hips out of alignment, leading to a stooped look and lower back pain.
Your muscles need to be fully warmed up before you perform static stretches, so never do them at the start of a workout. To avoid injury, don't pull too hard when you stretch, and don't 'bounce' the muscle under tension.