It's a mirror object put on top of the floors. Murano at TSR has some (I think they're pay), and there might be other versions too. There are also wall versions.
They're resource demanding, so if you've got a crappy computer, heavy lots, or a slow game with reflection turned on, you might not want to use them. They're not recommended for gameplay, but more for photoshoots.
Thanks for the feedback! I'm just certain I used to have a floor set of wooden floors, that had that effect, without it beeing a demanding addon. But it's a while back, maybe I'm just imagining it was glossy xD