Shutter Speed is the amount of time that the shutter is open. The length of time that your image sensor ‘sees’ the scene you’re attempting to capture. Shutter speed is measured in seconds – or in most cases fractions of seconds. The bigger the denominator the faster the speed (ie 1/1000 is much faster than 1/30). To “Freeze” action (sports, children playing, etc.), use a higher shutter speed. For lower light situations, a slower shutter speed may be better, but may cause blurring.
This photo was from a few weeks ago at the Wheel of Death during the Renaissance Festival in Tampa, FL. "Freezing" the guy in mid air at 1/800th of a second at f/3.5 ISO 1250.