Mass is a property of an object, a cloud of gas, a puddle of liquid, or a lump of goo, and doesn't necessarily change when the object moves to a different location or environment. It depends only on the object itself.
Weight is completely a product of what else is in the nearby location,so it can easily change. It's the result of the gravitational forces between two objects . . . the first one with the constant mass, and another one that also has mass. The forces between them, and therefore the weight, depend on both masses, AND on the distance between them.
