This configuration allows the site to extend a site subscription (not storefront subscription) by the set days within this configuration after normal subscription expiration before the subscription is actually expired. For example if a site subscription is set to expire on the 1st of the month and this configuration is set to 3 days that subscription would not expire until the 4th of the month allowing the subscription owner a little grace period before their subscription (and possibly their listings if you have the software set that way) expires.

This configuration is a subscription plan item within subscription based price plans and can be found in the COST SPECIFICS page of as shown in the screenshot below: