A oxidizer is a chemical which likes to give away its oxygen from another chemical (the reducer). Many fireworks and explosive contain set amounts of oxidizers and reducer which when light, produce alot of heat and gasses. For example, black powder is a mixture of potassium nitrate, charcoal and sulfur. Potassium nitrate is the oxidizer which like to give away its oxygen to the reducers, charcoal and sulfur. When this happens the chemical reaction occurs and produceres alot of heat and gas.