[nicw]

As pageld said, it's just a title. At an established startup, the product manager is generally the one translating the founders vision into a product. The founder might have an idea or see a need, but the product manager is the one who researches the market, does the competitive analysis and determines the feature set. Then they get it built and launched.

They may not do all the work, but they're the ones covering all the little details: making sure an email goes out when a new user signs up, reviewing that copy, watching retention, etc.

You'll see a lot of job descriptions for product managers reference 'Mini-CEO', which is pretty apt.