XMP Extensible Markup Platform
XMP files are text files which describe the contents of a file. Usually used to store data about photos. They follow a XML format can be opened with any text processing application, but it will make most sense with Adobes software.
File Juicer will extract text from any file type, and if you drop an image with XMP data on File Jucier, it will find that too. Currently saves it as a text file with the other text-like data found in the image.
More about XMPAdobe