Gravity is an invisible force that pulls all things toward the center of the Earth. That's why when you throw a ball up in the air, it always comes back down, and no matter how high you jump, you can't fly away.
This force is everywhere around us — it's why water pours downward, fruit falls from trees, and heavy objects stay on the ground instead of floating in the air. The heavier an object is, the stronger Earth's pull on it.
Far from Earth, like inside a space station, gravity is very weak, so astronauts and everything around them float instead of falling. That's what makes it amazing to watch them floating around inside the station.
