Speed is actually instantaneous speed, the speed an object is moving at that very instant.
But average speed is the total distance traveled divided by the total time.
Let's say you travelled 10 miles in 2 hours, your average speed would be 10 divided by 2, which is 10 miles / hr.
But during those 10 miles you may be accelerating, decelerating and be travelling 1 mile/hour and 20 miles/hour at another point. But your average speed would be total distance / total time. So your instantaneous speed can change throughout that period of time.
