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, which forms magnesium oxide:
Magnesium + Oxygen ⟶ Magnesium oxide
Mg + O ⟶ MgO
In this reaction, magnesium gains oxygen, so it is an oxidation reaction. We can also say that the magnesium has been oxidised.
On the other hand, metal oxides can lose oxygen. For example:
Magnesium oxide ⟶ Magnesium + Oxygen
MgO ⟶ Mg + O
In this reaction, magnesium oxide loses oxygen, so it is a reduction reaction. We can also say that magnesium oxide has been reduced.