Atherosclerosis is a condition in which fatty deposits called "plaques" build up in the lining of the blood vessels. As plaques get bigger, the blood vessels get narrow. This means the blood vessels cannot carry as much blood as before.
Reproduced from: What Is Atherosclerosis? National Heart, Lung, and Blood Institute. Available at http://www.nhlbi.nih.gov/health/health-topics/topics/atherosclerosis/.