Inertia is a fundamental property of matter that describes an object's resistance to changes in its state of motion. According to Newton's First Law, an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity unless acted upon by an external force. This principle explains why it is difficult to push a heavy object or why a moving vehicle takes time to stop. Inertia is directly related to an object's mass; the greater the mass, the greater the inertia. This concept is crucial in understanding various physical phenomena, from everyday activities like driving a car to complex systems in astrophysics and engineering.