In short, this error means that there are elements in your SSL secured pages that are being accessed from a non SSL secured page. A typical example of this is an image that resides in a non-SSL protected directory.
For example, a page that is loaded securely (via https), contains an image tag within the source code such as <img src =http://www.yyy.com/image.gif>. In this case the image is being called absolutely using the non-secure (http) protocol and will thus cause this warning.
This type of warning can be solved by:
1.) Moving the image file to the portion of the site that is https secure, e.g., https://secure.yyy.com/image.gif; or
2.) Making the links relative to the root directory by adding a backslash before the file name as in the following example: <img src="/image.gif">. This would translate to both <img src="https://www.yyy.com/image.gif"> and <img src="http://www.yyy.com/image.gif">.
This error can also be caused by Flash code.
As an example, this line of code will cause problems when trying to view over https
As you can see, the flash code base is being loaded from a non-secure location.
Macromedia has documented a solution: