Cardiac fibrosis commonly refers to the excess deposition of extracellular matrix in the cardiac muscle, but the term may also refer to an abnormal thickening of the heart valves due to inappropriate proliferation of cardiac fibroblasts.[1] Fibrotic cardiac muscle is stiffer and less compliant and is seen in the progression to heart failure. The description below focuses on a specific mechanism of valvular pathology but there are other causes of valve pathology and fibrosis of the cardiac muscle.

Reference
Wiki: Cardiac fibrosis