During open-heart valve surgery, the doctor makes a large incision in the chest. Blood is circulated outside of the body through a machine to add oxygen to it (cardiopulmonary bypass or heart-lung machine). The heart may be cooled to slow or stop the heartbeat so that the heart is protected from damage while surgery is done to replace the valve with an artificial valve.
The artificial valve might be mechanical (made of man-made substances). Others are made out of animal tissue, often from a pig.