A reducer is a chemical which likes to grab oxygen from another chemical (an oxidizer). 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. Charcoal and sulfur are both reducers which like to steal the oxygen bound to the potassium nitrate (the oxidizer). When this happens the chemical reaction producers alot of heat and gas.