What is Newton's First Law of Motion?

This is often called the Law of Inertia. It states: An object at rest will stay at rest, and an object in motion will stay in motion, unless acted upon by an external force.

Think about being in a Danfo. When the driver suddenly slams the brakes, your body jerks forward. Why? Because your body was in motion and "wanted" to keep moving forward even though the bus stopped. That resistance to change is Inertia.