In case you haven't been following the Superman books, here's the nickel catch-up.
Brainiac came to Earth, Superman beat him and disabled his ship. In the proces, he freed Kandor, the lost city of Krypton.
What Superman didn't know was just about anything about Krypton or Kryptonians, and its been a rocky ride since. Kandor took to the other side of the solar system with a new world (using that nifty crystal technology you remember from Superman: The Movie and Superman Returns). They've dubbed the world New Krypton.
DC has put together a snazzy video trailer for the upcoming story arc. Here.