The oil which is extracted from olives is used mainly for cooking.

extracted 是动词 extract 的过去分词形式，在句中作定语修饰 oil。extract 由前缀 ex-（表示"向外"）和词根 tract（来自拉丁语 trahere，意为"拉、拖"）组成，字面意思是"向外拉出"，引申为"提取、榨取"的意思。