Metals react with oxygen to make metal oxides. For example, magnesium burns in air with a bright white flame and leaves a white powder of magnesium oxide. Copper heated in air slowly turns black as copper oxide forms on its surface.
The word equation is metal + oxygen → metal oxide. For magnesium the balanced symbol equation is 2Mg + O2 → 2MgO. Every magnesium atom has joined with an oxygen atom, so the mass of the solid goes up when a metal is burned in air.
These reactions are examples of oxidation. In terms of oxygen, oxidation is the gain of oxygen. The metal gains oxygen, so the metal is oxidised.
The opposite process is reduction, which is the loss of oxygen. When a substance loses oxygen it is reduced. For this topic you only need to say which substance gains oxygen and which substance loses it.