XMP files are tiny data files of about 8kb each which can be created by Adobe Photoshop or Lightroom. They hold all of the raw processing settings, as well as star ratings, colour labels, and so forth.
Sending XMP files over the internet is a very fast way of returning the corrected files. You send us raw files (or small DNG copies of) by FTP, and after we’ve processed and corrected the images we generate XMP files and zip them into one file on the FTP server which are then a tiny download.
Once you receive the XMP files, you unzip the compressed file which contains them all (by double clicking) and copy them into your folder of related raw files. Then, following the instructions provided, Adobe Bridge or Lightroom can read the settings, and your files are updated using the new settings provided.
We just need to know which version of Photoshop or Lightroom you’re using.