Weight is is when something presses down due to gravity acting on it. When people go into space they float about because they are weightless. They are the same people, but in space there is no gravity acting on them. So something has weight only when gravity acts on it.

Normally, of course, we don't think about it that way, and just ask 'How much do you weigh?'. That is easy to answer because we stand on a machine with a spring in it and a dial turns to tell us what it is. Our weight (our body acted on by gravity) becomes a force that pushes against the spring.

Weight is measured in kilogrammes (kg) by scientists.