Hi gordong,
This message means that there is an image referenced in html within your description that is located on your computer. Since the image is located on your computer, you would be able to see it in your listing, while anyone else who views the listing will not be able to see the image.
To find out which image is the culprit, click the "Switch to text mode" link above the editor, then, hold down CTRL and click "F". Then, enter "C:" in the find field and click "Find". The image located on your computer will most likely begin with C:, which will become highlighted by the find command.
To remedy this issue, upload the image located on your computer into the listing using the "Freetiva Image" button(it has an icon of a yellow mountain on it).
I hope this helps=)
-Mike