A man, though slowly, can bring a heavy barge on the water into motion, if he pulls by the rope tied to it. He is not able to do that with heavy body, lying on earth, even though his body noticeably less than the weight of the barge. Why?
On the barge in water influences viscous resistance, proportionally to velocity, that is why with motion with low velocity friction force will be small. On body lying on earth influences sliding friction force, which does not depend on velocity and is equal to F=μmg, if body moves, and applied force to the body, if it is in quiescence (Newton's third law). Minimum force needed to budge a body is equal to F=μmg.