I read an awesome post on Coding Horror entitled Are you an Expert. There is a great quote that really exemplifies what product ownership means.
Being an expert isn't telling other people what you know. It's understanding what questions to ask, and flexibly applying your knowledge to the specific situation at hand. Being an expert means providing sensible, highly contextual direction.
A product owner has to be so in touch with the product, that they can ask the right questions. If the right questions aren’t asked, a team will soon be working on tasks that were simply on the list, not tasks that are important.