Metals react with oxygen to form metal oxides. Magnesium burns in air with a bright white flame and leaves a white powder, magnesium oxide: 2Mg + O2 → 2MgO. Copper heated in air slowly turns black as it forms copper oxide: 2Cu + O2 → 2CuO.
In both reactions the metal gains oxygen. A reaction in which a substance gains oxygen is called oxidation, so we say the metal has been oxidised. The opposite change is reduction, which is the loss of oxygen. A substance that loses oxygen is reduced.
Because oxygen from the air joins the metal, the oxide has a greater mass than the metal you started with. No atoms are lost. The extra mass is the oxygen that has been gained.