>>12858755https://en.wikipedia.org/wiki/Surface_roughnessIntuitively, it's the fact that the surface, when viewed on a fine scale, differs from the surface as viewed on a large scale. A ball might appear spherical, but it is actually bumpy. The "ideal" surface is a sphere, it could be described by an equation like x^2 + y^2 + z^2 = r^2, or as the set of points a fixed distance from its centre, or whatever, but the real surface isn't actually that simple.
It isn't really the roughness per se that causes the friction, but the fact that the two surfaces have a large number of areas where they are in contact, and it's at these areas they feel friction: rougher surfaces have more such areas. Lubricants actually have more total area in contact with the surface, typically being a liquid they fill all the gaps, but the lubricant-to-surface friction is very low relative to the surface-to-surface friction, so the total friction, the sum over all of the forces imparted by the surfaces rubbing together, is now lower.
