What does CPO in left corner mean in a car ad?

Probably means, 'Certified Pre Owned'. Which is a higher level used car, more thorough pre-inspection, better warranty, etc. Varies by brand dealership.

A certified pre-owned car or CPO is a type of used car. Most often late-model, they differ from other used cars by having been inspected, refurbished, and certified by a manufacturer or other certifying authority. They also typically include an extended warranty, special financing, and additional benefits.