The superman exercise is a core strengthening movement. It works the lower back muscles and the erector spinae muscle that runs the full length of the back on both sides of the spine. This is a great exercise to counterbalance crunches and sit-ups. To have a strong posture and reduce chances of back pain, you want to have a strong core on all sides of your body.
To perform the exercise you lie on your stomach and lift your arms and legs into the air as high as possible. You want to keep your arms and legs as straight as you can, but not completely locked out. As you raise them both simultaneously, exhale and squeeze your back muscles. It is important to go slow and deliberate.