Shutter Speed is the amount of time the shutter allows light to enter through the aperture, exposing the film or digital sensor.
Shutter speeds are measured in seconds or fractions of seconds. Faster speeds are better for scenes with motion that you would like to capture with a high degree of sharpness. Slower speeds are good for intentionally blurring motion or capturing static scenes in low-light conditions.
See also: Shutter Speed and f-stop Tradeoffs