Isaac Newton has three laws which explains the behavior of motion. His second law of motion states that force required to move an object is equal to the mass of the object multiplied by the acceleration of the object. Simply written, F=ma. This law explains that objects with more mass require more physical force to move. If enough force is applied to an object it will accelerate. The amount of force necessary to accelerate an object is dependent on the objects mass.
Your car runs out of gas along the side of the road. You will need to push it to the gas station that is 100 meters away. Your car has a lot of mass (it is heavy) so it takes a lot of force to push it. At first it is very difficult to get moving, however, once you get it moving (once you get it to accelerate) it becomes a little bit easier to push. If your car was smaller or if you were pushing it downhill it would require less force from you.