Mass is the amount of matter in an object, measured in kilograms. The change in an object's motion depends on the net force and on the mass. For the same object, a bigger force makes a bigger change in motion. For the same force, a bigger mass makes a smaller change in motion. This is the idea behind Newton's second law. Mass is not weight: weight is the force of gravity pulling on a mass, so weight changes from Earth to the Moon while mass stays the same.