What Does The Error Message "This page contains both secure and non-secure items." mean?

This is a client side message most likely caused by a reference in your HTML to a non-secured item.

All references in the secure page need to point to “https://”. The easiest way to accomplish this is to use relative links instead of absolute links. Commonly overlooked items include:

Images
Linked stylesheets
Frames, iFrames
Objects (such as Flash, Shockwave,…)
Scripts
Any of these HTML elements can trigger the warning message. If you are using resources from a third party, you may need to contact them for instructions on using their product on a secured web page.

IFrames can also cause this message. For more information on the IFrame issue follow this link:
http://support.microsoft.com/default.aspx?scid=kb;en-us;261188

Another source of this warning message is the use of “javascript:OpenURL();” in an anchor tag. For more information regarding the use of Javascript in anchor tags follow this link:
http://support.microsoft.com/default.aspx?scid=kb;en-us;269682