Sometimes I just don't get DC: Superman as Nightwing

Despite being the King of Nerds, I am not the Emperor of Nerds and even my uber-nerd knowledge, while vast, has its limitations.

Take for example this action figure of Superman as Nightwing. It has something to do with the Bottled City of Kandor but I'm not really sure.

I also don't understand how this was even popular enough to get made into an action figure.