All objects resist changes in their state of motion. All objects have inertia.

The tendency of an object to resist changes in its state of motion varies with mass.

The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion.

Suppose that there are two seemingly identical bricks at rest. Yet one brick consists of mortar and the other brick consists of Styrofoam. Without lifting the bricks, you could give the bricks an identical push in an effort to change their state of motion. The brick that offers the least resistance is the brick with the least inertia - and therefore the brick with the least mass (i.e., the Styrofoam brick).