The validity of the SSL certificate is about to expire. When a web browser encounters an expired certificate, the browser will normally present the user with a warning message indicating that the certificate has expired. Some browsers will continue connecting to the site after presenting the user with the warning, while others will prompt the user with a dialog box requesting their approval to proceed. These warnings are extremely confusing for the typical web user, and cause most users to question the authenticity of the site they are attempting to view.
Contact your Certificate Authority to renew the SSL certificate.