Conceptually I understand that mass is a measure of the amount of stuff present in an object, while weight is a measure of the force of gravity applied to that object. An object of a given mass will have a bigger weight on Earth than on the moon because Earth’s gravity is stronger. But… mass is determined by weighing an object on a scale. And there is a simple mathematical conversion between grams (mass) and pounds (weight), implying that they’re just different units for the same concept. So what gives? (Also this is a question that applies to so many fields, I had no idea which flair to apply.)
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As others have said, weighing an object on a spring or digital scale measures the weight (down force) that is applied to the scale and tells you the mass provided you are somewhere with the same gravity as the scale was made for. If you took the same scale and put it on the moon or mars, it would incorrectly tell you that your mass is a lot lower than it actually is, likewise, if you stood on the scales in an elevator and read out while it was accelerating, it would incorrectly read a higher mass because it is actually measuring down force and converting it to a mass reading
Note the a balance scale doesn’t have this problem as it compares 2 objects so both are affected equally when gravity changes.
Part of the problem is that we are a bit casual about mixing terms for weight and mass. Strictly speaking, your weight in the SI system should be measured in Newtons (units of force) and you mass in kilograms (units of mass) but we don’t for historical reasons and because so long as we don’t have to have equipment that measures in varying gravity, it doesn’t really matter.
It’s also worth noting that converting from grams to pounds isn’t converting mass to force – it’s converting SI to customary (imperial) and unfortunately, the imperial system uses pounds for both force and mass so unless you specify which you mean it can get confusing
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