Newtons I law:
It also knew that "law of inertia". which states that an object at rest remains at rest and an object in motion, it continues its motion until unless the object experiences the external force( net force).
This means the object is at rest always, until some other force acting on it to disturb, similarly if the object is in motion it continues its motion until some other force acting on it to take the object to rest position.
The net force means the vector sum of all the forces acted on an object.
Tendency of an object changes of its motion when acted up on by a net force is known as inertia. And the inertia is directly proportional to mass of the object