A product key refers to a unique 25 character alphanumeric code that is required to activate Microsoft Office 2013 after installation. The product key and its subsequent use in product activation allows Microsoft to verify that Office 2013 was legally purchased and that it has not been used on more computers than allowed by the software license.
The product key is often required when Office 2013 is being installed for the first time or when a subscription is being renewed. If Office was purchased at a retail store, the product key can be found on a product key sticker place on the packaging. If Office was downloaded and installed from the Internet, then the product key is usually sent along with the email receipt. Users whose PC came with a pre-installed version of Office can find the product key on the certificate of authenticity or a product key card within the packaging.
In most cases, all that is required to active Office 2013 during re-installation is to sign in to the account page and select install. However, users are still advised to keep a copy of their product key in case it is required for re-installation. Users who wish to renew their subscription must purchase another product key in order to reactivate Microsoft Office.