Has it moved?

Watch a shadow for ten seconds. It does not seem to move. Check again later. It has moved.

Position — where a thing is. The shadow's position changed, so it has moved.

Reference point — the thing you compare a position with.

In motion — the position changes compared with the reference point.

At rest — the position does not change compared with the reference point.

Example: a bag sits on a bus seat. Compared with the bus, the bag is at rest. Its distance from the bus does not change. Compared with a tree by the road, the bag is in motion. Its distance from the tree changes.