What is relative speed between two objects?
The relative speed is the difference in speeds.
If I am in a car traveling 60 mph and the car next to me is traveling 80 mph, from my perspective, I'm basically not moving and the other car looks like it's moving at 20 mph. The other driver's relative speed is 20 mph from my perspective.
Now if I'm the other driver, it looks like the first car is going backwards at 20 mph, relative me. Hence, the relative speed of me is - 20 mph for the other driver.
Technically, all speeds are relative, since "relative" just means that we are moving compared to someone in a different perspective. You can be relatively stationary (like I feel in my own car) or you can be relatively moving (like a 60 mph car relative the ground).