As described at Yahoo Answers, your camera's ISO setting controls how sensitive the camera is to light. A lower ISO requires more light to properly expose a photo than a higher ISO. Thus, a setting of ISO 200 needs twice as much light to properly expose a photograph as ISO 400. Your camera's ISO setting is important for sports when there's not a lot of light to maintain the fast shutter speeds necessary to clearly photograph action. Boosting your camera's ISO to 800, 1600, 3200, or even 6400 can dramatically increase the ability to properly expose your sports shots with a fast enough shutter speed to stop the action. The downside to higher ISO settings is the introduction of more noise in your photos as ISO increases. Regarding noise, not all camera's are created equal. Some camera's introduce unusable noise at ISO settings as low as 1600. Others produce very good photos at ISO 6400. If you want to take action shots indoors or in other low light situations, having a camera that performs well at higher ISO settings can be very useful.
The photo below was shot at the Pontiac Silverdome (the old home of the Detroit Lions). While the lighting in the Silverdome is very good, this shot still required an ISO setting of 4000 to properly expose this photo at aperture f/2.8 and a shutter speed of 1/1250s.