Mass is a measure of how much ‘stuff’ something is made off, while weight is measure of how strongly the force of gravity is pulling it down. The mass of an object is the same whether it is on earth or on the moon, but because the moon has less gravitational pull, it would weigh less on the moon than on earth.
There is a joke that going to space being the best way to lose weight, because you become weightless, but what people really mean when they say they want ot lose weight is that they want to lose mass.
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