Shutter-Speed is how long the film or sensor is being exposed to the light.
Inside the camera body, is a light proof curtain. When we press the shutter button, the curtain opens and closes again.
If an object moves from Point A to Point B from the time the shutter opens until it closes again, it will blur. Simple as that. If this shutter duration is so fast that the subject doesn't move, it will appear to have been frozen in time.
Like Aperture, Shutter-Speed also provides us with a visual impact to our photograph.
A blurred photo illustrates motion, like a running child or a waterfall. While freezing something in lid-air creates an unworldly effect. We never see a bird stuck mid-air during flight, yet we've all seen the pictures which show that.