If the two participants in an SMTP transaction both agree to it/support it, the transaction can be encrypted using SSL or TLS. This adds a level of security to email since message content is no longer being sent in plain text across the Internet. An RFC describing the extension to SMTP can be found here: http://www.ietf.org/rfc/rfc2487.txt.
The daily reports provide statistics for those messages that were encrypted using SSL or TLS and those that were not. Since all servers do not support Secure SMTP and both sides of the transaction must support it in order to encrypt the message, many of the messages passing through the IronMail will not be secured by SSL/TLS. I think there is a Secure Web Delivery option for the appliance that can be configured to allow secure message delivery to recipients whose mail systems don't support Secure SMTP.