Monthly subscriptions do renew automatically one month after your billing date, until you elect to cancel. Similarly, annual subscriptions will automatically renew one year after they are initiated until cancelled.
For example, if you initiate a monthly subscription on the 5th day of a month, it would automatically renew at the same price on the 5th day of the next month unless cancelled in-app. If you only want to try a subscription for one month or do not want for it to renew on your billing day, make sure to cancel.
For instructions on how to cancel, please see this article: How do I cancel my plan?