Mirror Mode is basically the mode in Mario Kart where all of the tracks you race on are mirrored. So basically, you know all of those strategies and the map layouts you made in your head? Well you better throw that all away or else you’ll fall in lava, water, or get to the off-road and lose your chances of winning!
TL;DR: The tracks but mirrored. Forget the tracks you’ve been strategizing unless you wanna lose the first place.