A secure microsite uses SSL (Secure Sockets Layer) to encrypt the data that is passed between a visitor's browser and the web server. This type of microsite is the preferred type over basic microsites. Normally, data sent between the browser and the webserver is transmitted in plain text. SSL encrypts the data so it cannot be intercepted by a third party, making it ideal for microsites that will involve the exchange of sensitive information, like passwords.
Any https:// domain is using SSL to encrypt the connection. Most modern web browsers include a visual indicator showing that the connection is secure. For example, Eloqua's login page uses SSL, as indicated by the green lock symbol, and Oracle Corporation (US) information in the left side of the URL bar.
Setting up a secure microsite requires acquiring and configuring an SSL certification. Review the documentation for creating secure microsites.