Restless Legs Syndrome is an overwhelming urge to move the legs usually caused by uncomfortable or unpleasant sensations in the legs. The sensations have the following features:

Occur during periods of inactivity
Become more sensitive in the evening and at night

Are relieved by movement of the limb
Often cause difficulty staying or falling asleep, which leads to feelings of daytime tiredness or fatigue

May cause involuntary jerking of the limbs during sleep and sometimes during wakefulness (not the common jerking most people have as they fall asleep)

Up to 8% of the US population may have this neurologic condition. Many people have a mild form of the disorder, but RLS severely affects the lives of millions of individuals.

I gather that none really knows what causes RLS but one logical hypotheseis/maybe even a theory is that it is due to some sort of problem with your spinal cord or nerve roots.