Flange Focal Distance is distance from a lens mount flange to focal plane, and this distance is the same, irrespective of the actual focal length as a 10mm lens should focus the rays to the same physical location as does a 600mm lens.

But different cameras have different flange focal distance that makes it impossible to make some adapters. For example, you can't make an adapter between Arri PL and OCT-19 mounts. Or you need to use optical elements like in infinity-allowing M42 to Nikon-F adapter.