The Weight of something tells us how heavy it is. The Mass of something is similar, but slightly different. Mass is used a lot in astronomy.
This is because the surface gravity is less on the Moon - as Nelly is only too pleased to find out (see cartoon).
However, astronomers want to be able to say how much of something there is without having to worry about exactly where it is at the time. They therefore use the Mass of things.
The mass is the weight of an object on Earth divided by "g", the value of the surface gravity.
Since the weight doubles on a planet with a surface gravity of 2 x g, when you divide the weight by the surface gravity to get the mass on that planet, it is the same value as on Earth.
Therefore, mass is a bit like weight, but does not depend on where you are in the universe.