In this reaction Magnesium removes Oxygen from Copper( II) Oxide . This shows that Magnesium is more reactive than Copper

Magnesium Oxide is a white powder.Magnesium gains Oxygen and is therefore oxidized.Copper (II) Oxide losses Oxygen and is said to be reduced. The removal of oxygen from a substance is called reduction. Addition of oxygen to a substance is called Oxidation. The substance being oxidized is called a reducing agent and the one being reduced is called the oxidizing agent.