Mass is constant where weight can change due to gravity.

like if your mass is 500 lbs when you are it outer space where your mass = 500 but
you weight = 0.
 where weight is

weight = mass x acceleration due to gravity

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The Mass of an object doesn't change when an object's location changes. Weight, on the otherhand does change with location. 
Mass is a measurement of the amount of matter something contains, while Weight is the measurement of the pull of gravity on an object.