Force is a physical cause that changes (or tends to change) either the size or shape or the state of rest or motion of the body.
The forces which act on bodies when they are in physical contact are called contact forces.
When a body moves over a rough surface, a force acts on the body in a direction opposite to the motion of the body along the surface of contact. This is called the frictional force or the force of friction.
When a person moves toward the right on a road, the force of friction acts on him toward the left. This force resists his motion on the road.