It means any aircraft transitioning over the area at high altitude isn't impacted, because they're too high to care.

It is a ground and "everything near the ground" stop. Meaning low altitude helicopters and private aircraft have to consider it, even transitioning, but realistically commercial aircraft not taking off/landing in the area won't.

FL180 is 18000 feet, meaning that flights OVER don’t need to divert.

It means that you have no business being below FL180 or 18,000ft to enter this airspace.

That it limits local flights but not international ones as they fly higher.