The property of matter that causes it to resist changes in motion is inertia. An object at rest tends to remain at rest, while an object moving in a straight line tends to keep moving at constant speed unless an external force changes its motion.
Inertia is described by Newton’s first law of motion, also called the law of inertia. Mass is the quantitative measure of an object’s translational inertia: a more massive object requires more force to produce the same acceleration. Inertia is not a separate substance or force acting against motion.
Everyday friction often hides inertia. A rolling ball eventually stops because friction and air resistance act on it, not because inertia makes moving objects slow down. When a car brakes suddenly, passengers lurch forward because their bodies tend to continue moving at the car’s original speed. Rotating objects have rotational inertia as well; their resistance to changes in spin depends on how their mass is distributed around the axis.