This is an exercise working out the muscles around the waist. If you do this regularly you can strengthen the deep back muscles supporting the lower section of the spine.
Lie down into prone position on the ground, tuck in your hip in such way that your belly button should elevate from the ground. Your legs / feet are on the ground at all times, your arms are straight next to your ears, your head is the extension of your torso. From this starting position lift your chest and arms from the ground, hold the position then while you are holding it pull your arms to your ears (try to hold your elbows as high as you can) then stretch them forward and then let yourself down to the ground. Repeat the exercise until fatigued.