A money-spinner is a product, a show, a book, a shop, or any line of work that quietly (or loudly) makes a lot of money. It is a British, informal, friendly word for a moneymaker — the thing that pays the bills and then some. People say it with a grin: “That little app became a real money-spinner.” It is the thing, not the person. Though actually, it is a bit of a picture-word: a spinner is something that spins, like a wheel spinning thread, and here the joke is that the business keeps spinning out cash. You would not put it in a bank report; you would say it over coffee. American English is more likely to say moneymaker or cash cow for the same idea.