How does Newton’s first law of motion describe inertia?

Isaac Newton’s first law of motion, often called the law of inertia, states that an object will remain at rest or continue moving at a constant speed in a straight line unless acted upon by an external force.1

This law describes inertia as the fundamental tendency of a body to maintain its state of motion. While common experience might suggest that objects naturally come to a stop, Newton’s formulation clarifies that such deceleration is actually caused by unbalanced forces, such as friction or air resistance.1

In this context, inertia is a passive property of matter that resists changes to an object's velocity—specifically its speed or direction—when an agency attempts to alter its state. Consequently, in classical mechanics, there is no essential distinction between being at rest and moving at a constant velocity; both represent a state where no net force is acting upon the body.21