The importance of Newton's first law of motion is that:
it mentions force
it defines inertia
Newton's first law states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an unbalanced force. This is also called the law of inertia because inertia is the property of any object by virtue of which it retains its state of motion. The importance of Newton's first law of motion also lies in the fact that it mentions force and gives us a qualitative idea about it.