Average and instantaneous speed are both ways of measuring the average distance that an object travels in a given amount of time. The difference is the amount of time we are using to find the speed, and the ways in which they are calculated.
First; speed is typically measured in units of meters per second (m/s) in physics, meaning how many meters are traveled in a certain number of seconds. Average speed is typically applied in a circumstance where the speed measurement is not consistent - such as finding the average speed of a car over the course of a road trip, and taking into consideration the times when the car is at a a full stop with zero speed, such as at a stop light, and when the car is at top speed on the highway. The average speed will need to take into account the total time, and the total distance, which will mean the average speed is less than the top speed, and higher than zero.
Something that moves with constant speed has no real need of an "average" measurement, because there is no variation to measure.
Instantaneous speed is more of a concept than a measurement. We cannot freeze time and observe that an object has some kind of intrinsic "speed" that is a part of it at that moment. Since speed is a measure of distance and time, we must be able to measure both in order to come to a value. However, we can calculate what the instantaneous speed would be if we know other values; for example, if an object is moving with constant speed X, then we know that its instantaneous speed at any point in its path must be X as well.
The attached link provides the example of a speedometer telling you your instantaneous speed while in a car, but even a speedometer requires a timeframe gap in order to operate correctly, and it also cannot function when the car is in reverse, indicating that its ability to measure instantaneous velocity is simply another instance of humans creating a tool that functions well within the terms of its job, but reveals its limitations when inspected from a scientific perspective