Longdesc,detailed description, is an attribute for images that are being used to link to a separate document. This linking is being done for the image in the image archive.
Switch over to the Image archive.
Beneath the alternative Link to more information about the image (longdesc) you click on browse.
Here you can link the image to more information, e.g. a file, web page or an external link. The common link dialog will open.