It’s a cake baked with the batter poured over a layer of fruit and brown sugar, usually pineapple. As it bakes the fruit caramelizes, and when it’s done you flip it upside-down on a tray with the fruit facing up, hence the name. It’s pretty fucking tasty

I’m guessing you’re joking, but the truth is that I know far less about upside down cake than umeboshi. I basically only know that I’ve eaten some when I was a kid, and you turn it upside down when it comes out of the pan, and there’s toppings there. I think the baker of upside down cakes likes to do the presentation in front of people.

Here’s Wikipedia.