Multiple sclerosis is a chronic, inflammatory disease affecting the central nervous system that causes a hardening and destruction of the myelin sheaths of neurons. The myelin sheath is a fatty, protective layer of the neurons (cells of the brain and spinal cord) that help the latter carry electrical signals to allow for flow of information among the cells.
The name "multiple sclerosis" arose because of the multiple scars (scleroses) that form on the myelin sheaths as they are gradually destroyed by the disease.