Have you subscribed to an app via your iPhone or iPad? Perhaps it was a free trial subscription that required a monthly fee to continue. It can be one of Apple's own services, such as Apple Music or Apple Arcade, or a third-party app, such as a password manager and a music streaming service.
If you use the service on your Apple device, you can log out there. That sounds handy, but how does it work? Here you can read how you can cancel a subscription on your iPhone, iPad or computer.