Myelin sheath is a protective coating of fat and protein that wraps around the axon like links of sausage.
The myelin sheath also serves to increase the velocity with which electrical impulses travel though axons. Those axons that carry the most important and most urgently required information have the greatest concentrations of myelin. If your hand touches a painfully hot stove, for example, the information regarding the pain is passed through axons in the hand and arm that have a relatively thick coating of myelin, speeding the message of pain to the brain so that you can react instantly.