Inertia, in mechanics, is the resistance to changes in velocity and it is measured by the quantity mass. As the mass increases it is necessary to increase the force either to start a movement from rest or to accelerate an ongoing movement. A train has more inertia than a car, it's harder to put a train in motion or to accelerate it.
Inertia is an inherent property of matter. (we might call it laziness) Matter - objects - do not move on their own accord. it need s a push or a pull. FORCE IS THAT WHICH IS NEEDED TO OVERCOME THE INERTIA OF MATTER.