The vulkan renderer is based on the DXVK open-source project (slightly modified to be more stable with Train Simulator). It replace the old DirectX 9 renderer of Train Simulator with a Vulkan renderer.
In some cases this will greatly improve framerate (especially framerate stability) on some routes that are using a lot of complex geometry / 3D vegetation. This wiki page explain quite a lot of things: https://wiki.rwenhancer.com/doku.php?id=core:vulkanrenderer
Vulkan renderer is disabled by default so it's up to the user to try it or not (it is easy to try and do not replace any TS base files) We will see if this Vulkan renderer will become a must for Train Simulator in the future.
Like RWE 2 shaders effects, this can work even if you are playing Train Simulator without RWE 2 HUD and gameplay features.