An action potential is a spike of both positive and negative ionic discharge that travels along the membrane of a cell.  The creation and conduction of action potentials represents a fundamental means of communication in the nervous system.  Action potentials represent rapid reversals in voltage across the plasma membrane of axons.  These rapid reversals are mediated by voltage-gated ion channels found in the plasma membrane.  
The action potential travels from one location in the cell to another, but ion flow across the membrane occurs only at the nodes of Ranvier. As a result, the action potential signal jumps along the axon, from node to node, rather than propagating smoothly, as they do in axons that lack a myelin sheath. The clustering of voltage-gated sodium and potassium ion channels at the nodes permits this behavior.