A base is a firm support that you can build something on. A subscription base is the list of people who have subscribed to what ever you are publishing. If you publish a magazine, and you only have one thousand subscribers, it would be foolhardy to print six million copies of your magazine. You probably won't be able to sell them all. But if you have five million subscribers (a strong subscription base), then it would be realistic to print six million copies of your magazine. Sales of six million copies will probably be supported by a base of five million subscribers.