1) The heart muscle requires a certain amount of energy just to survive; it requires increasing amounts of energy to perform more and more work.
2) When you are resting the heart is still pumping blood in your body, however when you start exerting like walking to the bathroom body requires more blood to be pumped by the heart i.e. more work for the heart. Lets say you go upstairs that requires more blood flow than previously and finally when you start jogging or sprinting the blood flow in the body can easily go up from almost 5 litres per minute to 15 -20 litres per minute.
3) To perform the work (pump the blood) the heart also requires increasing amounts of energy, nutrients and oxygen that comes to it via the blood supply of the heart: coronary arteries
4) Even when the coronary arteries are narrow by as much as 60% there may not be significant trouble for a patient. However, as the narrowing progresses in severity- the blood flow and energy supply to the heart progressively turns critical and the patients suffer from angina at severe exertion, at moderate exertion and then even at rest.
5) And if the energy supply is cutoff completely then the heart can not even survive: portions of the heart actually die.