It's called a road, it's called a rainbow road. It is a road that you go. Rainbow Road is the last course in all of the Mario Kart games (this one's from Mario Kart 7) and was made a stage in Super Smash Bros 3DS. You will know when you get there.
Rainbow Road basically takes the track element that the previous F-Zero stages have used and applies it to the racing game series that Nintendo still cares about. Also, the Shy Guys are back from Brawl's Mario Circuit, because I doubt anyone (besides BurgandBurb) really wants to see the Honey Queen again.