mass
Mass is the amount of 'stuff' in an object, how much matter it contains. It is what makes an object hard to accelerate and what gives it weight when gravity acts. A brick has more mass than a feather. The everyday question it answers is simply: how much is there?
Mass is a scalar quantity (just a number with a unit, no direction), measured in kilograms (kg) in SI. It plays two roles: as inertial mass it measures resistance to acceleration in Newton's second law, a = F_net / m; as gravitational mass it sets how strongly gravity pulls, giving weight W = m g. Experiment shows these two masses are equal. Crucially, mass does not change from place to place: 2 kg on Earth is 2 kg on the Moon or in deep space.
Mass is often confused with weight. Weight is the force of gravity on the mass and changes with location; you would weigh about one-sixth as much on the Moon. But your mass, the amount of matter, stays the same. Kilograms measure mass; newtons measure weight.
A 2 kg bag of sugar has a mass of 2 kg everywhere. On Earth it weighs about 20 N; on the Moon it weighs only about 3 N, but it is still 2 kg of sugar.
Mass stays constant; only weight changes with gravity.
Mass and weight are different quantities with different units (kg versus N). A bathroom scale is calibrated for Earth's gravity, so it can read mass only because g is roughly constant here.