1.4 - Roughness

The Roughness input controls a Material's roughness. Rough Materials scatter reflected light in more directions than smooth Materials, which is how blurry or sharp a reflection is (or in how broad or tight a specular highlight is). A roughness value of 0 (smooth) results in a mirror reflection, and a roughness value of 1 (rough) results in a diffuse (or matte) surface.

Roughness is a property that will frequently be mapped on your objects in order to add the most physical variation to the surface.