For those unsure what RTX is, or what you're looking at in the video, RTX uses ray-tracing- an old existing tech that's been utilized in games for some time now for other tasks, it essentially sends out lines (rays) from a point to an object and then calculates whatever is needed from there (in this case determining what the surface is and how it should respond to the light or what the player can see and if it is reflective). How I understand this to be working is they're sending out rays from the player's view, which then bounce around as necessary and create the accuracy for the reflections. Then every light source also has it's own rays being cast out to create accurate shadows as well (the shadows are highlighted better in a Tomb Raider demo, but the reflections are IMO where this tech really shines). Someone else might be able to provide a more technical explanation as I only understand it at a surface level :P
Edit: The other thing that it has been shown to do is accurately reproduce how light should travel through a transparent object. So if it should refract (such a through a glass ball) then it will do so properly with this tech... Nvidia also claimed it is very "simple" for devs to implement the tech which could help with adoption (but who knows how accurate such a claim is).
Still very much early days for this tech as the price for the new cards that support it is ridiculously high and puts it beyond the reach of what most people are going to be willing to spend. As well the early reports on framerates are suggesting 60fps and 1080 which for PC gamers is likely going to be a downgrade in resolution OR fps (or both). But it's definitely an exciting development when you consider in the "near future" this is likely just going to be the norm on both PC and console. Especially if AMD launch a competing technology.
I personally think in this demo the windows are too reflective, but that sort of thing is probably going to come down to specific engines and how developers decide to use the tech.
(Also mariomguy if you are reading this I do still plan to try your demo. When I put my PC back in place last week it had developed some issues randomly that I've had to sort out so not had a chance to try it yet.)