A law of physics states that "an object at rest tends to remain at rest, and an object in motion tends to remain in motion." Scientists call these tendencies inertia. Inertia is a way of measuring how hard it is to change the momentum of an object, whether that's getting it to speed up or getting it to slow down. That depends on how much mass the object has. Big heavy things (things with a lot of mass) have more inertia than light things. You have to push a bus harder than a scooter to get it to move.
If something has a lot of mass, it's also hard to get it to stop moving. If the bus was moving fast, you'd need good brakes to get it to stop. Because the bus has more mass than the scooter, it would be a lot harder to stop the bus. That's also inertia - inertia's a way of measuring how hard it is to get something to stop moving, too.
To find out more about physics, check out these books from Amazon.com or from your library: