Apparently, the measurement refers to heart wall, or possibly the wall that separates the heart's lower chambers. When the wall becomes pathologically thick (as opposed to an athletes wall dimensions...non-pathological), that can effect the amount of blood pumped into circulation with each heartbeat as well as irregular heartbeats.
A heart operation, called a myomectomy, is sometimes done if there is a significant blockage of blood flow. In this operation, some of the excess heart muscle is removed. A heart transplant may be needed if there are severe symptoms in spite of treatment with heart medicines/myomectomy. A heart transplant may also be needed if there are abnormal heart rhythms that cannot be safely treated by a cardioverter-defibrillator