Metals react with oxygen in the air to form metal oxides. These are redox reactions.
For example, let’s look at the reaction between magnesium and oxygen from the air, which forms magnesium oxide:
Magnesium + Oxygen ⟶ Magnesium oxide
2Mg + O2 ⟶ 2MgO
In this reaction, magnesium gains oxygen from the air, so it undergoes oxidation. We can also say that the magnesium has been oxidised.
On the other hand, metal oxides can lose oxygen, releasing it into the air. For example:
Magnesium oxide ⟶ Magnesium + Oxygen
2MgO ⟶ 2Mg + O2
In this reaction, oxygen from the magnesium oxide is released into the air, so the oxygen undergoes reduction. We can also say that the oxygen in magnesium oxide has been reduced.