what do we mean when we say that motion is relative?

Answer 1

What we mean is that it is impossible to say that something is moving without explicitly or implicitly referring to something we regard as being still.

In situations in our daily lives, we regard the earth as fixed, and objects as moving relative to the earth. Suppose you and your companion are on a moving train, relative to one of you the other is not moving at all; however, in colloquial usage, we say that both of you are moving relative to the earth.

Closely related to this is the notion of reference frames. A reference frame is a coordinate system (x, y, z axis if we use cartesian coordinates) that is fixed to whatever we regard as being still, and which we use to describe the position of whatever is moving.

Relative velocities are computed by examining what happens when the reference frame is changed. All material objects change their velocity when that happens. For example, if we examine your motion relative to a reference frame fixed to the train, your velocity is zero. This of course, is "common sense".

This segues rather nicely into special relativity. People (Michaelson and Morley) found through an experiment, that a beam of light does not alter its velocity when reference frames are changed - which of course is counter intuitive, but that's just it. The only way to explain that "paradox" is to say that clocks run at different speeds in different reference frames!

Answer 2

When two object are moving in space their motion is measured from the same starting point are said to be relative to the same measuring point.
The Stating point is the zero point of measurement. Science calls this measuring point a frame of reference.
So motion of objects to have any relative meaning must be measured from the same local space.
Therefore, if the motion of two objects are relative to the same starting point of local space then they are said to be relative to each other.

Answer 3

Like relative to the Earth. If you're just sitting somewhere, you aren't moving, relative to the Earth. But relative to the universe, we could be moving lightyears per second. Get it?

Answer 4

it is relative to the earth, every single thing in this world has inertia, meaning it does not move unless moved, thats why we get pulled back when accelerating in a car, so motion is relative to earth.