Sometimes when offering a new paid membership or subscription, you want to let people sign up now, but not charge them until some fixed date in the future, because all the content isn't going to be available until then. Or just as a promo.
The trial period option allows you to accomplish just this. By telling this system how many days after you purchase before you want to begin charging the customer.
When the trial period ends, your customer will be charged for the continued subscription if they haven't already canceled. If you have a minimum commitment period that period will begin now preventing them from canceling on their own.
You may also want to set up a notification email letting them know their subscription is going to end. Below are the step by step instructions for creating such a notification.