Nitrous is a slang term for nitrous oxide (N2O), commonly used by drag racing classes like Pro Street, Top Sportsman, and Pro Mod. Nitrous oxide is an oxidizing agent used to increase an engine's power output by allowing for faster burning of a fuel (usually gasoline). Nitrous can be used with alcohol in the mud racing categories.
Nitrous oxide is an oxidizer, not a fuel. It carries more oxygen to the engine, allowing for faster burning of the fuel and generating more power.