Yeah, it's not totally obvious. It's an old phrase and I've never really liked it. A similar one is "trader prince", which is pronounced a lot like "traitor prince", which of course means something totally different.

Anyway, it's usually a prince that's also a merchant. Historically, it refers to merchants who aren't really princes or even any kind of nobility, but they get rich as fuck by trading across the kingdom. In the case above, the story focuses on a family that wasn't originally noble, but got there after a very peculiar trade monopoly.