If I remember correctly!!!
For this expression to be used you are talking about obtaining the velocity at any absolutely minute moment in time on a changing velocity.
The change in velocity is acceleration. The thing is; acceleration could also be changing giving you a double whammy!
If you were to plot the changing velocity against time then you would get a curve. The tangent to this curve at any point is the resulting acceleration.
Check this to make sure it is correct: the instantaneous velocity is a lead up to the process of differentiation and or integration in Calculus.