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A greater value produces rougher or wider results that spread across the surface from the edge. At 1.0 it resembles a nearly diffuse surface and at 0.0 it's so thin it may be hard to see depending on the shape of your surface. Most objects will be realistic somewhere in between these values. Texturing this value may give you interesting effects like smudges, greasy fingerprints, and worn surfaces.

Normal/Bump

Here is where you supply a signal, either a texture or procedural pattern, to create a bump to the surface to "fake" surface details like small bumps or scratches. This means an artist doesn't have to model these tedious and often repetitious parts of a model.