A law of physics first suggested by the Uzbek scientist Ibn Sina about 1000 AD 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.
Ibn Sina saw that the reason real objects on Earth seem to slow down on their own is that friction with the air or the ground stops them. Later on, Galileo did experiments to show that if you rolled a ball down a ramp it would go almost as far up the ramp on the other side, and the smoother the ramp, the farther the ball would go. Then Isaac Newton worked out the mathematics to prove how inertia worked.