How does Newton's first law of motion apply to the movement of objects in a state of inertia?

Newton's first law of motion states that an object at rest will stay at rest, and an object in motion will stay in motion at a constant velocity unless acted upon by an external force. In the context of objects in a state of inertia, this law explains that objects will continue to remain still or move in a straight line at a constant speed unless a force is applied to change their motion.