Myelin is an electrically insulating material that forms a layer, called a myelin sheath, around certain cells (typically around the axon of neurons). It is made by myelinating Schwann cells wrapping themselves around the axons of these neurons.
Myelin production usually starts around the 14th week of fetal development and after this is a long, grow increase that can last into our thirties i.e. myelination occurs well into adulthood.
Axons which lack myelin tend to conduct action potentials at a velocity of 0.5-10 metres per second. When myelinated, however, axons can conduct the same signals at a rate of up to 150 metres per second, between 15-300 times the previous speed of transmission. This speed increase is brought about by a reduction in the distance that conduction occurs in.
When myelination occurs there are gaps where conduction takes place (called nodes of Ranvier) as opposed to conductions taking place along the entire length of the axon. If all of the axon was insulated, current wouldn't be able to flow out of the axon and an action potential couldn't be generated.
Having nodes of Ranvier allows the current to flow passively along the short distance insulated by the myelin sheath to the next node. This form of current propagation is known as saltatory conduction.