So far we've talked about aperture, which defines how much light gets through, and ISO number, which determines the light sensitivity of the camera's sensor. Shutter speed is the third part of the exposure equation. It refers to how long the shutter remains open to allow light through. Shutter speeds are given in fractions of a second -- you'll commonly shoot at 1/500.
Shutter speed is tricky because slow shutter speeds don't just allow more light through, they can cause blurring. The entire time the shutter is open, light is hitting the sensor, and if an object (or the camera itself) moves during that time, the movement will show up as a blur. If you've ever seen one of those photographs of the stars at night showing the lines of their motion as the Earth rotates, that image was the result of a very long shutter speed recording the starlight over several hours.
You can reduce blur by using a tripod to hold the camera steady when you shoot, which lets you use slower shutter speeds, but if you're shooting sports and you don't want blurry players, that only helps so much.