What is nitro?
The term could refer to nitro oxide, commonly known as "laughing gas". Or it could mean nitro methane  a mixture produced via a chemical reaction between nitro acid  and propane
Commonly used in drag races, both are illegal in most countries for street use. You will recall scenes from the movie 2 Fast 2 Furious where a racer hits a button that activates a pump, and then everything on the road become a blur.
That is nitro oxide, which is a gas stored in a cylinder and pumped into the combustion chamber for times when extra power is needed. But because a lot of it is needed to create that short burst of power, racers cannot afford to carry huge cylinders of it. Also, incorrect use will result in a blown engine.
Nitro methane is a pre-mix used by drag racers, although some model cars are now made to run on the stuff.