Can someone clarify me what will happen if I disable this option in player settings? Quality of graphics will be worst? Is there any difference?
The DirectX API in use by the renderer doesn’t really have anything to do with graphics quality. That’s strictly a result of your choice of textures, lighting, shaders and models. DirectX 11 gives you access to a more uptodate Shader Model, including Geometry shaders and those that belong to the Tessellation engine, i.e. Hull shaders and Domain shaders. Those can be used to implement some effects that aren’t possible with DirectX 9, but they cannot be seen as an overall “graphics quality toggle”.
Think of it like a toolbox. Using DirectX 11 is like adding a screwdriver and sandpaper to a toolbox that only contained a hammer and a saw before. But that doesn’t automatically make your house prettier. It’s how you USE the tools that matters.