Unreactive metals such as gold are found in the Earth as the metal itself. Most metals are found as compounds, often oxides, and a chemical reaction is needed to extract the metal.
Metals less reactive than carbon can be extracted from their oxides by reduction with carbon. Reduction is the loss of oxygen. The metal oxide is heated with carbon, and the carbon takes the oxygen away. For example, iron oxide is reduced by carbon: 2Fe2O3 + 3C → 4Fe + 3CO2. In the same way, copper oxide is reduced to copper: 2CuO + C → 2Cu + CO2.
In these reactions the metal oxide loses oxygen, so it is reduced. The carbon gains oxygen, so it is oxidised, and it forms carbon dioxide. Carbon sits between magnesium and zinc in the reactivity series. Zinc oxide can be reduced by carbon, but oxides of potassium, sodium, lithium, calcium and magnesium cannot, because those metals are more reactive than carbon and need a different method.