Mass is the quantity of matter that a substance possess.
However, weight is a function of mass and gravity,i.e to say that weight of a substance varies as the gravity on different planets vary.Mass of a substance always remains the same because the amount of substance that the object possesses doesn't go anywhere and it always remains there as stated by law of conservation of mass. Whereas the weight of that same substance only varies due to the change in gravity (which is different on every planet and is constant for each planet.Gravity on earth is 9.8 m/(s*2). I hope that clears the concept. :)