If you would like to display an image description beneath the images you check the option "Show image descriptions" and select the metadata, the metadata needs to be created in the same way as categories described in step 1.
The difference is that you add a metadata type text field instead of the multiple selection.
Then you add the metadata value to the images by right clicking on the images in the image archive and select Properties. Beneath the tab metadata you add a text beneath Description.
The result will be an image description shown beneath the images which have had a description added: