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Confidence votes 317. Inertia is rotary momentum; an object's ability to continue spinning when a decelerating force is applied. Just as a heavy object takes more effort to stop from the same speed as a light object, so does a heavy object take more effort to stop from turning at the same speed as a light object. So assuming they are turning at the same angular velocity, the heavier one has a larger inertia.
Mass is important because of two major factors affecting how things move in space: inertia and gravity. The more mass something has, the more of both it experiences. That is why heavy things (things with a lot of mass) are hard to move. When an object is sitting still, it resists moving, and the more mass it has the more it resists. The amount of thrust needed to move something and how fast it ends up moving are both directly tied to its mass.