What does CPO stand for?

It means it is Certified-Preowned. Basically it means that even though it is a used vehicle it is as close to a new vehicle as a used one can be.

Certified pre-owned. Dealers do a manufacturer mandated 100 (or so) point inspection on the vehicle. Used vehicles with CPO designation usually sell for a bit more, but they typically come with some kind of manufacturer warranty as well.