What do we mean when we say that motion is relative

It means that all motion is "compared to ..." something.

-- When you're reading a book on an airplane, the book is not moving
compared to you, but it's moving at more than 400 mph compared to
someone on the ground.

-- When you're riding in a car, the driver isn't moving compared to
you, but he's moving at 30 or 40 mph compared to a person standing
on the curb.

You might ask: "But how fast is the book REALLY moving ?  How fast
is the driver REALLY moving ?"

The answer is:  There's no such thing as "really" moving.  ALL motion
is relative to (compared to) something.

If you want to touch off an explosion and watch somebody's face turn
red, green, and blue ... some rainy afternoon when there's not a lot
going on ... tell this to somebody who believes the Earth is flat and
is willing to fight to keep believing it.