Roughness is not inverted specular, it just the old way to make your pre-PBR work at least somehow.
Roughness is exactly what it says: level of roughness/smoothness of the surface. Rough surface tend to scatter light rays in ~random directions what gives blurry(diffuse) reflection to observer. ~Perfectly smooth surface(like mirror) gives you “specular” sharp reflection.
When you create your roughness map you should think about type of material surface you’re currently working with. Yellow plastic could be polished(low roughness), then scratched(medium roughness), covered with dirty fingerprints(high roughness) and so on. Think physically about your material, it’s not just set of flat textures anymore(Well, conceptually :p).
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