Two different quantities
In everyday speech "mass" and "weight" are used to mean the same thing. In physics they are two completely different quantities.
Mass
The mass of an object is the amount of matter contained in it.
Its international unit is the kg.
Weight
The weight of an object is the force pulling it towards the earth.
That is, the force applied to it by gravitational attraction.
The fundamental difference
Mass is something inside the object. Weight is something applied to the object.
So wherever you take an object, its mass does not change. Its weight can — depending on which body is pulling on it.
Because weight is a force
its international unit is the newton (N) — not the kilogram. The next page shows how to work it out.