For most apps on the App Store, your subscription will end at the end of your subscription period, and you will not be billed. If you’re on a free trial, access to your subscription content might end immediately. If this is the case, you’ll be given the option to continue your free trial until the end of the trial period.
For example: let’s say you subscribed to Amazing App+ for one month on May 25, and you cancel your subscription today. You will continue to have access to Amazing App+ until June 25, and you won’t be billed again.
You can manage your subscriptions here.