The definition of a subscription is an agreement you make in advance to receive something for a specific period of time. When you make an arrangement to have a magazine delivered every month, this is an example of a subscription.
So in case of World Creator, you subscribe yearly, to use World Creator. Once the subscription period has ended, you no longer can run and use World Creator (which means, you have to re-subscribe again).
During your subscription period, you have access to all new updates and versions of World Creator that we release.