Shutter Speed (SS) is the time it takes for the mirror to go up and down. While the mirror flips up, the sensor (the object that records the image) is exposed. When the mirror flips back down, the sensor stops recording the image. The longer the shutter speed, the more light that hits the sensor. Have you ever seen a picture that has a river or waterfall that the water looks this and wonder how its done?
Well it is done by having a slow shutter speed. Due to a slow shutter speed, the sensor records the water flowing and the final result is like the picture above. If you take a picture of a flowing river like this with a fast shutter speed, it will look dull and uninteresting. For sports, you generally use a fast shutter speed so there is no blur in your picture.
The general rule of thumb for taking pictures that have no blur in them is that you double the focal length you are using and use the reciprocal of that that. For instance, if you are at 200mm, your shutter speed should be 1/400. However, if you have image stabilization in your lens, you can use a slower shutter speed and still get a shot with no blur.
I will explain more about how/why to use a slow shutter speed in a post that will be coming soon that links everything together.
Hope that helps!