If you own a dairy cow, you go out and milk it every morning, take the milk, and sell it that day. (It doesn't quite work that way in the more modern, industrialized dairy farms, but keep the image in mind.) After you're done, you're x dollars richer and still have the cow to do it again tomorrow. in fact, your only material loss is some water and hay.
A "cash cow", then, is any product which does basically the same thing--produces a steady stream of income for its owner without subtracting any important assets in the process. Unlike manufactured goods, which cost money for parts and materials every time a new product is made, these products cost next to nothing beyond the initial investment in order to keep the money coming in.
Good examples include cult TV shows to which a network owns syndication rights, online software that requires a monthly subscription, computerized gambling machines, and patents on widely-used intellectual property.