If you have embedded captions into your pictures, for example with Picasa, my modification makes it so that they will be 'parsed' (interpreted by the computer) automatically while making the gallery.xml. Before, the captions always defaulted to 'Show picture full size' and if you wanted to edit them you had to do it through the wordpress interface. I made it so that before uploading you can put in captions with whatever software you like.
For now, it only recognises IPTC captions; there is also XMP, and I'm sure adding support for that too wouldn't be too hard - but it wasn't necessary for what I wanted to get done. AFAIK IPTC is most widely used - it's certainly what Picasa uses.