Nitro is a short way to say 'nitroglycerin', which is a kind of liquid that can explode very easily.
Nitro is often used in stories about action and danger.
Examples:
The race car uses nitro to go faster.
I don't want to touch the nitro because it might explode.
Nitro is very dangerous.
The nitro made the car go super fast!
With nitro, the rocket went to space in just a few seconds.
He poured the nitro into the engine and the car zoomed away.