Depending on numerous factors, including cholesterol levels, some of the arteries in the body can become partially blocked with plaques of cholesterol and fat. This narrows the arteries reducing blood flow to the area. This means that the tissue which that artery serves suffers from lack of oxygen and nutrients - slow starving of that tissue/muscle.
When this happens to the arteries of the heart, it can result in pain in the heart (known as angina), especially with exertion like exercise or strong emotions. Not everyone will have pain; some people will feel short of breath or light-headed. Angina is a risk factor for heart attacks and other heart conditions, so it is often treated through either medications or surgeries.
One such surgery is heart bypass surgery, also known as Coronary Artery Bypass Graft Surgery or CABG for short. CABG creates new pathways around the narrow or blocked artery so that more blood and oxygen can flow to the affected heart muscle - i.e. it creates a way for blood to 'bypass' the diseased artery.
Therapeutic Guidelines http://www.tg.org.au