Friction is a phenomena which comes into existence when one body moves over one body. it depends on the smoothness of the surface of friction and also of the weight of the upper body. it is also a natural build as if we drag an object on the road or in any surface, road/surface impart a frictional force on us. to keep the object moving we have to exert more force to over come the friction. Frictional force also justifies the law of static inertia.