If a car is moving in the same direction and with speed as you, then what can you say about the motion of the car?
The car is at rest with respect to you.
We see the world from our point of view, so the motion of another body depends on our motion i.e the motion of the observer as well.
Since the other car is also moving at the same speed and in the same direction as you, when you look at the other car it will appear as it is at rest.
So we can say that the other car is at rest with respect to you while actually, it is moving with respect to the ground.