Summary: Blood flow out of the heart to the body can become obstructed (blocked) because of the thickening in the heart wall.
Hypertrophy means that muscle cells become larger, and so the muscle itself increases in size. When you exercise, this happens to your muscles - so it's a desirable result. But hypertrophy in your heart muscle cells, as in HCM, can cause serious problems. "Cardiomyopathy" means a disease of the heart muscle cells.