Any motion is a relative motion because you are either in a reference frame or a non reference frame
a motion which we observe without being in motion is the motion with respect to the earth since the earth is not stationary

 when you are in any reference frame which is moving at a constant velocity with respect to the the moving direction . For example, you can toss and catch a ball in a moving train if the motion is in a "straight line" at constant speed(when some 1 pulls the chain then train will stop suddenly so u feel the motion (respective to the direction of motion )