The shutter speed is used to control how long the sensor is exposed to light. The faster the shutter speed, the less light it lets in. The slower the speed, the more light that gets to the sensor. This also works with the aperture and ISO settings.
It is also used to control motion. A fast shutter speed (such as 1/250th of a second) can be used to stop something in motion.
A slower shutter speed (such as 1/30th of a second), can be used to blur motion.
