What does Nitro track mean? I understand Retro track, tracks from older MK games but i'm not sure what Nitro means.

It means the 'newer tracks' that were added in Mario Kart 8, so basically Mushroom, Flower, Star and Special Cup.

There are some new tracks in the bonus cups too (egg, triforce, crossing and bell) but these don't count towards the 'nitro' tracks; they have have their own category called 'bonus' tracks