Shutter Speed is the amount of time the shutter is open. More light gets in the longer the shutter is open. Too much light, and your photo will be overexposed – white, washed out. Not enough light, and your photo will be underexposed – dark, no detail.
Combined with different aperture settings, your shutter speed can drastically alter the look of your photos. At night, you can use a long exposure to create light streaks. In the day, you can use a short exposure to capture a soccer player in action.
Take a look at the photos below and read the captions to get an idea of what different shutter speeds can do.