Epic Games, which makes Fortnite, also makes the Unreal Engine that powers it. (Remember when Unreal was more than an engine?) Epic just released the the first preview build of Unreal Engine 4.22, and among a great number of other changes and updates, the new build includes preliminary support for Microsoft's DirectX Ray-tracing (DXR) extensions to the DirectX 12 API.
Naturally, to make use of the shiny new graphics hardware, devs will have to be building games for DirectX 12. Likewise, you'll have to actually be building a game—this announcement is simply that developers who are making games using the Unreal Engine can now optionally target a DirectX-12-with-DXR platform.
We're big fans of Nvidia's GeForce RTX graphics cards around here, if only because they offer extremely flat frame-time graphs in existing games at little or no extra cost compared to similarly speedy Pascal hardware. As of today, it's pretty hard to figure the value of the "RTX" part of Nvidia's latest graphics chips because there's simply so little to actually do with them that you can't do on a previous-generation processor.
That could all change if DXR starts to gain traction. Support in big software packages like Unreal Engine is only one piece of the puzzle, though. Part of the reason that RTX hasn't had quite the visual impact we might hope in its current implementations is that the scenes in existing software have to be optimized for regular old rasterization, in addition to hybrid rendering. It's unclear if game developers will be willing to optimize their scenes for what is, so far, a vanishingly small part of the market.
Folks who are interested can hit the Unreal Engine forums to read about all the new stuff in the update.