It simply means that EVERYTHING on the page must be called using a secure server link.

Some carts will allow relative links (.../.../file.txt) but you may need to go through and
change ALL links in the code to secure, and place copies of all images and dependent templates
in a secure directory (depending on how your server is configured).

For example, if you are calling a 'footer template' from your unsecure area of the site,
it would be flagged by the server as an 'unsecure' item on the page. Usually the script
running the cart has secure/unsecure access, as long as you specify in your settings you are
running a secure cart and provide the secure url to the cart.

- Puamana