How does friction arise?
Friction is an interaction opposing relative motion, or the tendency for motion, between surfaces in contact. Microscopic roughness, adhesion, and deformation all contribute. Friction does not always point opposite to an object's overall motion.
Static and kinetic friction
Static friction adjusts to the required value while surfaces do not slip; it is not always at its maximum. Kinetic friction describes sliding contact. A simple dry-friction model makes the force proportional to the normal force, but the coefficient depends on materials, temperature, and surface conditions. Lubrication changes contact by introducing a fluid layer rather than eliminating every form of friction.
Useful effects and energy losses
While walking, a foot pushes backward against the ground and the ground pushes the person forward through static friction. Braking converts mechanical energy mainly into heat. Extremely smooth surfaces do not necessarily have less friction, because molecular adhesion can increase. Engineering therefore aims to retain sufficient grip where needed while reducing unwanted energy loss in moving components.