Gravity
Gravity is a force. It makes things move toward each other. On the Earth, we see that it makes things fall toward the ground.We say that things that are pulled by gravity have mass. The larger the masses, the bigger the pull of gravity. The farther apart the things are, the smaller the pull of gravity.
On the Earth, we can measure the pull of the Earth's gravity on soemthing as weight. The pull of the gravity of the Moon is only about 1/6 of the gravity of the Earth. So something that weighs 60 kilograms on the Earth would only weigh 10 kilograms on the Moon. It would still have the same mass, but the smaller gravity of the Moon pulls the mass less.
Things that are falling still have mass, but we can't measure their weight, so we say that they are "weightless". Astronauts and spacecraft in space can be weightless. They look like they are floating. Really they are falling in an orbit around the Earth.
They still have mass, so we still have to push them or pull them to make them go or stop. Astronauts inside spacecraft use their arms and legs to jump or to stop. When astronauts are working outside in space, they use very small rockets to move. Spacecraft use bigger rockets to move.
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