Myelin is a fatty substance that wraps the axons to increase the membrane resistance (by blocking the leakage through the ion channels). The passive current can flow through a piece myelinated axon without leaking until it reaches the nodes of Ranvier (French histologist) where myelination is interrupted. Here inward Na2 currents through the voltage-gated Na+channels cause another action potential which regenerate the passive current to flow through the next segment of myelinated axon. This process is repeated at every node until the signal arrives at the terminals. The signal travels through the axon at a speed about 120 m/sec.
