inertia
Inertia is an object's stubbornness, its resistance to any change in its motion. A loaded shopping cart is hard to get moving and, once rolling, hard to stop or turn. That reluctance to start, stop, or change direction is inertia. It answers: why is it harder to push a truck than a bicycle?
Inertia is measured by mass: the greater the mass, the greater the inertia, and the smaller the acceleration a given force produces. This follows from Newton's second law, a = F_net / m; for the same net force, doubling the mass halves the acceleration. Mass in this role is sometimes called inertial mass. Inertia is not a force; it is a property of matter.
You feel inertia every time a bus brakes and your body lurches forward: your body 'wants' to keep moving at the old speed. Seat belts exist to supply the force that safely overcomes your inertia. Newton's first law is often called the law of inertia because it is precisely the law describing this property.
Pushing an empty grocery cart, a gentle nudge sends it rolling. Fill it with heavy bottles and the same nudge barely moves it: you have added mass, and hence inertia.
More mass means more inertia: the same push produces less acceleration.
Inertia depends only on mass, not on weight or gravity. A massive object floating in space still has full inertia; it is just as hard to shove even though it has no weight there.