If a substance oxidizes, it combines with oxygen and loses hydrogen to form another substance, and if something oxidizes a substance, it causes it to do this.

Iron oxidizes to form rust. 铁氧化形成铁锈。

When you heat fat, it oxidizes easily. 脂肪在燃烧时迅速氧化。

If a chemical element oxidizes, it loses electrons (= very small pieces of matter with negative electrical charge) and if you oxidize it, you cause it to do this.