Why is there a difference between your weight on earth then your weight on the moon?

Mass is a universal unit. It does not change depending on your location because the amount of matter something is made of does not change from location to location. Weight is a much more relative unit. Weight is related to gravity as opposed to the amount of mass making up something.

On the moon has approximately 1/6 the gravity of Earth. Therefore, if you weigh 120 pounds on Earth you would weigh 20 pounds on the moon.

Because earth's gravitational pull is stronger than the moon's.