Somebody said they are different because of gravity.
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All stuff has mass. It is a basic part of being stuff. It is what scientists call a constant because no matter where the stuff is, it will always have the same mass.
Gravity is caused by mass. Why this is gets very complicated.
A mass sitting all by itself doesn’t have weight as we mean it. It only has weight when it is affected by being close to another mass. Technically, even a tiny mass creates a really tiny amount of gravity. But for any mass we can move around by hand, the amount of gravity it makes is so teeny tiny that we can ignore it most of the time.
When we weigh something, we are not measuring mass directly. What we are doing is measuring how much local gravity is pulling on that mass.
(It is easy to measure how much the Earth is pulling on a 1 Kg test mass, but extremely difficult to measure the minute amount the test mass is pulling on the Earth.)
This is why a given mass sitting on a scale here on Earth will weigh more than the exact same mass sitting on a scale that is on the Moon. The Moon is smaller, so there is less mass. Less mass means the Moon creates less gravity than the Earth can. So it doesn’t pull on the test mass as strongly. Weight equals mass X gravity.
The reason we can get mass and weight so easily confused is because the amount of gravity the Earth generates is used as a standard reference for all kinds of measurements. Basically, we decided that Earths gravity has a value of 1. So a 1 kilogram mass, multiplied by 1 (Earths gravity) equals 1 Kg of weight. That same mass on the Moon weighs 1 Kg X .166 (the percentage of Earth gravity the Moon can generate) = 16.6 grams.
Because in our daily life, 1 Kg of mass always equals 1 Kg of weight, we never think about what is really going on when we weigh a mass.
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