Someone please explain.
An example would be lovely.
Velocity is defined as (change in position) / (change in time). At any particular moment, a moving object has move a direction and a speed, thus a velocity. That is the instantaneous velocity, the one at that instant. The change in time is part of a limit approaching zero.
Now suppose you know a car drove 200 miles non-stop in 2 hours, didn't change directions or anything, but it wasn't driving at the same speed during the whole trip. If you take the change in position and divide by the total time, you will have an average velocity for the trip but that won't tell you anything about a particular time during the trip, just the trip as a whole.