Average velocity is calculated over a period of time. For instance, let's say the initial position is zero meters. The final position is twenty meters. If it took five seconds to travel twenty meters, then the average velocity is four meters per second.
Instantaneous velocity is calculated at an instance of time. An example would be calculating the velocity at twenty-one meters at a time of exactly three seconds. The instantaneous velocity would be seven meters per second.
In both cases, derivatives and/or limits can be used to make velocity calculations.