From MedlinePlus Genetics
Familial hypercholesterolemia is an inherited condition characterized by very high levels of cholesterol in the blood. Cholesterol is a waxy, fat-like substance that is produced in the body and obtained from foods that come from animals (particularly egg yolks, meat, poultry, fish, and dairy products). The body needs this substance to build cell membranes, make certain hormones, and produce compounds that aid in fat digestion. In people with familial hypercholesterolemia, the body is unable to get rid of extra cholesterol, and it builds up in the blood. Too much cholesterol increases a person's risk of developing heart disease.

People with familial hypercholesterolemia have a high risk of developing a form of heart disease called coronary artery disease at a young age. This condition occurs when excess cholesterol in the bloodstream is deposited on the inner walls of blood vessels, particularly the arteries that supply blood to the heart (coronary arteries). The abnormal buildup of cholesterol forms clumps (plaques) that narrow and harden artery walls. As the plaques get bigger, they can clog the arteries and restrict the flow of blood to the heart. The buildup of plaques in coronary arteries causes a form of chest pain called angina and greatly increases a person's risk of having a heart attack.

Familial hypercholesterolemia can also cause health problems related to the buildup of excess cholesterol in tissues other than the heart and blood vessels. If cholesterol accumulates in the tissues that attach muscles to bones (tendons), it causes characteristic growths called tendon xanthomas. These growths most often affect the Achilles tendons, which attach the calf muscles to the heels, and tendons in the hands and fingers. Yellowish cholesterol deposits can develop under the skin of the eyelids and are known as xanthelasmata. Cholesterol can also accumulate at the edges of the clear, front surface of the eye (the cornea), leading to a gray-colored ring called an arcus cornealis.  https://medlineplus.gov/genetics/condition/familial-hypercholesterolemia