Average speed is calculated by overall distance divided by the time taken. Constant speed is when the speed does not change during the object's travel.
The green line represents a fast and steady speed. The red shows that the object was moving at a steady speed, stopped, and then returned back to where it came from. The blue line portrays that the object is moving at an accelerating pace. If you wanted to find the speed at any given time from the graph, just take the distance at that point and divide it by the time, also at that point.
When you are in the car, the speedometer may read 55 mph. But, five minutes later it may read 49 mph. This is an example of decreasing speed. If you turn on cruise control, it will help you maintain a constant speed.