What exactly is the Apple subscription? I don't believe I signed up for this.

Apple has a number of subscriptions you can sing up for. They absolutely cannot start by themselves as the system requires various steps to confirm the subscription. However, you may have downloaded a free app from the App Store that had a Free trial,. and forgot to cancel it. In most cases Free trials automatically renew into paid subscriptions if you don't cancel them before they renew.

You can follow the steps here, to find and cancel any active subscriptions you may have: Link-> How to find and cancel a subscription from Apple - Apple Support