Secure Mode provides an SSL (Secure Socket Layer) connection between your web browser and the JISCMail server.
You need a web browser that supports SSL encryption - all the major browsers do.
It is primarily intended to be used just prior to accessing the LISTSERV web interface, via links such as 'List Management' or 'Register Password' on the JISCMail homepage. It will ensure all login and cookie details are sent over an encrypted connection.
Most web browsers will display a padlock icon on the status or address bars, and you can check for https:// in the address bar of your browser too.