It Means "Certified Pre-Owned". When you buy a second hand BMW from a BMW dealer it either has no warranty, some 30 day warranty from the dealer, the remainder of the original owner's warranty 36/36 or 48/50 or 60/50, or - 'best' -it can be Certified Pre-Owned or 'CPO'. If it is CPO, then the dealer has allegedly inspected it fully in accordance with an official checklist from BMW that the mechanic must sign; this qualifies it to be sold 'CPO'. The benefit of this to the consumer is that the warranty that ends at 50K miles gets extended to cover major items with (I think) a $50 per incident deductible.
Not all parts or services are covered but there are other benefits like Roadside Assistance etc.
If you're buying a used BMW, no matter what, CPO or not, I think you should always have an independent expert inspect it for collision history etc. I bought a CPO 2001 740i Sport once from a dealer near Philadelphia. It was CPO'd and I had the mechanic's checklist with signature. I got hit about 3 weeks after I picked it up by an idiot teenager in a Jeep. The damage was minor but when the high end body shop I had it towed to looked at the car they called me to say the car had been in a crushing rear end collision and the A-pillars were bent and parts had been spot welded back together. The mechanic had signed off that he had checked for all this but there it was.
The dealer claimed no knowledge of this and refunded my money but the two cars I had traded were gone along with the tax break for trading against the purchase- so BE CAREFUL out there! [Oh, by the way, yeah I did run a CarFax report on it before buying it and there was no indication of the accident or any salvage indication at all. You have to read their guarantee really closely too to see that they do not guarantee and of that info]