The friction coefficient is a dimensionless quantity and its value depends both on the typology of the materials in contact and on the treatment of the surfaces (which determines the surface roughness and therefore the interaction forces between the atoms of the materials in contact).
Friction is a macroscopic phenomenon of resistance, due to the roughness of the material surfaces. It is a phenomenon that occurs everywhere in the contact between two material bodies, even if only minimally, both in static (static friction) and dynamic (dynamic friction in relative motion) conditions, giving rise to a resisting force called friction force.
The effects of friction manifest a dissipation of mechanical energy (or kinetic energy) that is transformed into heat, which reduces the efficiency of the movement but in some cases is of fundamental utility, for example, if it is not looking for a free movement but an adhesion or relative motion control.
The friction coefficient is equal to the ratio between the friction force developed between the two bodies and the force that keeps them in contact.
The static friction coefficient is always greater, or at most, equal to the dynamic friction coefficient for the same surfaces in contact. From the microscopic point of view, it is due to the interaction forces between the atoms of the materials in contact. This implies that the first detachment force required (i.e. to cause the bodies to begin to slide in relative motion) is greater than that required to keep them in sliding.
The static friction coefficient is equal to the tangent of the maximum angle that can be reached between the two forces before one of the two bodies begins to slide along the other (friction angle).