The plane takes off, and it takes off slightly sooner than it otherwise would have.
The runway will go backwards as fast as the plane moves forwards. The vast length of the moving runway will naturally entrain some air and will drag said air past the plane thus slightly increasing the planes airspeed over what it would have been with a static rnuway.
The runway can only move backwards if the plane is moving forwards.
Imagine the question is asked but the plane does not touch the ground. The plane flies over the runway at 600 km/h. The runway moves backwards at 600 km/h. Does the plane fall out of the sky? Of course not. Wheels down, wheels up, ground speed is irrelevant.
Now, imagine the same question but with a car and ask does the car move? Or just as much fun, does the road move?
Imagine that the car starts to move at 1 km/h. The road starts to move backwards at 1 km/h. Now the car is spinning it's wheels to stay in one place. But now it's velocity is zero. Ergo the runway matches that and the car starts to move. So the road ...
etc