A tourniquet is a piece of cloth or cord tightened around a limb to reduce blood flow to a body part. In some cases it is used while amputating a limb, but in this case it was used to stop the bleeding from a missing limb. Many men were mutilated and lost limbs in Vietnam. Wounds like this were often caused by grenades, booby-trapped equipment and landmines.
If a tourniquet is left on too long, the tissue below it will die from lack of oxygen.