The Key: Frames of Reference
The concept of "rest" and "motion" is relative. It depends on your perspective, or your frame of reference.
Example:
* Imagine you're sitting on a train. You're at rest relative to the train.
* However, you're in motion relative to someone standing on the ground outside the train.
A Body in Both Rest and Motion?
It's possible for a body to be at rest relative to one frame of reference and in motion relative to another. Here's a great example:
* The Earth: The Earth is at rest relative to us, standing on its surface. But, it's in motion relative to the Sun (orbiting it) and the center of the Milky Way Galaxy (rotating around it).
Conclusion:
There's no single, absolute "rest" or "motion." It all depends on what you're comparing the body to.
