Constant Speed is When something is going the same speed throughout its motion. Say when someone is driving they were going forty-five miles while they are driving and their speed never increased they kept going the same speed throughout. Its like Constant Speed to a car driving the same speed , so they will be going the same speed throughout.
Variable speed is when you are going a speed and you change your speed. For example if you were walking and you were walking 6 miles per hours, and then you change your walking pace to 10 miles per hour you then have variable speed by changing. A comparison is Variable speed to change is pace.
Average speed is when your going the same speed throughout, you add up all your speed and divide and then that shows what your speed s. An example is when a person is playing basketball you add up all the points and divide by the number of games and then that shows you how many points total were made. A comparison is Average is to basketball games.
Instantaneous speed is the speed at a given moment or time. An example is when your driving a car and you look at the dash board, and you see your speed. Its speed to given moment.