What is relative motion?
Relative motion is the velocity of one mass with respect to another, even though both may be moving with respect to an observer.
An easy way to think about this is if you are standing next to a highway and two cars go past, one at 50 MPH and the other at 52 MPH. Even though the motion of both cars is fast compared to you, the driver of each car looks at the other and sees that the relative motion of the two cars is only 2 MPH. Keeping relative speeds low on our highways is an important key to highway safety!