Mass Stays Constant
Mass is the amount of matter in an object, measured in kilograms. It does not change with location — a 10 kg bag has 10 kg of mass on Earth, on the Moon, and in deep space.
Weight Depends on Gravity
Weight is the force gravity exerts on that mass, measured in newtons (or, loosely, in pounds). Because the Moon's gravity is about one-sixth of Earth's, the same 10 kg bag weighs about 16 N on the Moon versus 98 N on Earth. In everyday speech people say 'weight' when they mean mass, which is fine until you leave Earth or do physics.
The Everyday Approximation
On Earth, a 1 kg mass weighs about 9.81 N, or about 2.2 pounds. Converter tools labeled 'weight' are really converting mass between kilograms and pounds — a distinction that only matters in precise scientific work.