Inertia is the resistance, of any physical object, to any change in its velocity.
Examples of Inertia
Shaking a bottle of ketchup. When bringing the bottom down, the suddenly stopping it, inertia is what causes the ketchup to come out of the bottle.
When a car is abruptly accelerated, drivers and passengers may feel as though their bodies are moving backward. In reality, inertia is making the body want to stay in place as the car moves forward.
A car that is moving will continue even if you switch the engine off.
Inertia and Mass
The larger the mass of the object, the greater the inertia. For example, when a running car and a train brakes, the car stops easily, but the train does not stop easily.