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This is a mechanism in neurons that allows impulses to travel along the axon much faster than without it(up to 150 meters per second). A regular nerve signal propagates along the axon like so (the signal moves from left to right):
First Action Potential:
-   -   +   +   +   +   +   +   Charge on outside of membrane
------------------------------ Membrane
+ | +   -   -   -   -   -   -   Charge inside the membrane
  V
  Na+

Second Action Potential:
  K+
  ^
+ | +   -   -   +   +   +   +  Charge on outside of membrane
------------------------------ Membrane
-   -   + | +   -   -   -   -  Charge inside the membrane
          V
          Na+
And on, down the whole length of the neuron.
When the axon has a myelinated sheath(called a Schwann Cell) with gaps in it(each of which are called a node of Ranvier), propagation jumps from node to node. This is called saltatory conduction from the latin word saltare, "to leap." It looks like this:
First Action Potential:
O -- OOOOOOOOOOOOO ++ OOO Outside of membrane
----------------------------- Membrane
  +|+              --     Inside of membrane
   V
   Na+

Second  Action Potential:
 K+
 ^ 
O+|+OOOOOOOOOOOOO -- OOO   Outside of membrane
-----------------------------  Membrane
  --              +|+     Inside of membrane
                   V
                   Na+
The "OOO" outside the membrane is the Schwann cell, and the area between where the charge is, is the node of Ranvier. Since charge jumps from node to node, it doesn't have to slowly diffuse down the length of the axon. Without saltatory conduction messages to and from your lower extemities would take much longer, and larger life forms would have a large disadvantage.

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