A balance in a lab measures the weight of a substance or object.
Weight is the mass of the body x the gravitation pull on the mass of the object.
So the mass of the object can be found by dividing the weight by gravitational constant.
The gravitational constant on earth is 1. so if a balance says that a substance weighs 300g then its mass is also 300g on earth because 300/1 = 300.
Hope that helps :)