what is meant by the motion is relative?

Answer 1

what that means is is that the motion is referenced to another moving body. that is, if your car is doing 70 mph and another car is going 80 mph, it is moving 10 mph relative to your car.

Answer 2

The best example I ever read:
If you are moving along at a steady speed in an automobile and you drop a small, heavy ball out of the window, you will see it fall straight to the ground. However, someone standing on the sidewalk will see it moving forward with the vehicle as it falls to the ground in a parabolic curve.
If we remove the car and the earth, what is it's true path in space?
As you may have already guessed, it has no "true" path in space - it only has a path relative to an observer, and that path varies depending on the motion of the observer.
This is true of all motion.

Answer 3

They mean the motion (velocity) given from a particular frame of reference.

Example:
Two cars (Car A and Car B)
Car A is moving at 40 miles/hr
Car B is moving at 60 miles/hr

1) Car A is heading directly at Car B:
From Car A, the relative velocity of Car B is 120 miles/hr

2) Car A is following Car B:
From Car A, the relative velocity of Car B is 20 miles/hr