The amount of stuff that something is made from
Mass is different to weight. Mass is how much stuff something is made from whereas weight is how much force that mass is pulled downwards by. For example, a 60kg man will weight 600N on the Earth (where gravitational field strength = 10N/Kg). If the same man goes into space his mass will still be 60kg as he still has the same amount of stuff in him, whereas his weight would become zero.