That's called windowsill (or "podokonnik" in Russian, lit. "under window") and they're very common in Europe. On the vid, it must be one of the finest I've seen though.

Still, I didn't think this of all things would confuse the American. Reminds me of another window meme lmao (pic8.co)

End grain blocks of wood glued together, filled with sawdust and glue, sanded and polyurethaned to make a window sill.

That's a beautiful windowsill. It's crazy to think of the amount of work condensed in that short video.

Well this would be a backsplash if it was vertical, so maybe it’s just a splash. Or a windowsplash.

This now belongs to the cat.