In this mode the camera constantly refocuses on a moving subject. If the subject is moving at a constant rate towards or away from the camera the focus system will 'predict' the new focus distance. This is especially successful when AF point selection is set to auto (the camera can use all 45 focus points to track a moving subject). In this mode the focus confirmation light only indicates if focus can not be achieved (it blinks).
If you've ever tried to track a moving object with a long tele and keep a single focus point on it you'll understand the benefit.