This can be clearly explained by an example
The best example for this would be a chair on earth.
When, we see a chair on the floor, according tp us it is in rest.
But, now let's zoom out, see it from the space.
Now, is the chair still at rest ?
No! Its moving as the earth moves.
It means the chair was rest with relative to the earth.
But with relative to sun or any other body in space, it in motion.
So, a body at motion at rest or motion is relative.