The heart is an amazing organ. It beats thousands of times each day, every day, for your entire life. In the process, it pumps about 5 million gallons of blood through your body!
If one of these outer arteries gets blocked, it causes a heart attack. A blockage like this is normally caused by fatty deposits that build up in the heart's arteries over the course of many years. Everything you hear about fat in the diet, cholesterol, coronary artery disease and "clogged arteries" is focused on this problem -- it turns out that blocked heart arteries and the heart attacks they cause are a leading killer in the United States.
When one of the heart's arteries gets blocked and a person has a heart attack, one common procedure is to perform heart surgery and sew in a new piece of blood vessel to bridge over (bypass) the blockage. In many cases, the surgeon will fix not only the immediate problem, but also other arteries on the heart that are starting to look blocked. If the surgeon repairs three of the arteries, it is called a triple bypass. If four arteries are repaired, it's a quadruple bypass.
The blood vessel used to create the bypass is taken from the chest or the leg -- the body has several redundant vessels that can be removed without doing harm.