It's hardware accelerated support for tracing rays efficiently. That can be used to implement a variety of rendering approaches that make use of tracing rays, including traditional Whitted ray tracing, path tracing and all sorts of hybrid approaches.
To be more precise: RTX (like OptiX before it) makes no particular assumptions about the applications. It concerns itself mostly with fast ray/scene intersections while user code in the form of very specialized shaders form the actual rendering algorithms around that core. That makes sense because constructing acceleration structures and the intersection tests themselves tend to dominate the runtime for these algorithms unless some particularly fancy shading is applied to the surfaces.
