Instantaneous velocity is calculated at a specific point and average velocity is calculated over a distance.
Let's say I was taking a ride in my car to another city, and I drove for an hour. If the other city was 60 miles away, that means my average velocity would be 60 mph. But, during that trip, I got caught in a traffic jam, where at some point in time I was going 20 mph. That would be an example of instantaneous velocity.
I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.