Some objects are more difficult to accelerate than others - A physics book resists accelerating more than a pencil, for instance. In other words, some objects have more inertia than others.
Mass measures inertia. The mass of an object tells you how much the object resists accelerating. In this sense, mass measures the quantity of matter in an object.
On the other hand, many people prefer to face the truth (as we know it) - even if the truth turns out to be complicated, confusing, or even mystifying. If you are one of these people, click on the button below: