let's say you take a trip over the river and through the woods to grandmother's house, covering a distance of 150 miles in 3 hours.

your average velocity for the entire trip is (150 miles)/(3 hrs) = 50 mph

now, if you traveled via car, does that mean your speedometer always pointed at 50 mph? ... of course not. The speedometer reads the instantaneous speed, or the magnitude of velocity, at every instant of time.

as the measured time interval gets smaller, the average velocity over that measured time period approaches the instantaneous velocity at a specific time within that interval of time.

see the difference?