What is OPML?
OPML, or Outline Processor Markup Language, is a standardized XML file format for describing outlines. The OPML file format is supported by many outlining packages, and also by many mind-mapping programs.
The reason it can be so useful when combined with MultiMarkdown is that it allows you to easily restructure longer documents with a drag and drop interface. For example, if you're writing a book with multiple chapters, and multiple sections and subsections within the chapters, you can use an outliner or mind-mapping program to rearrange your writing to change the structure easily.
MultiMarkdown Composer can read and write MultiMarkdown OPML files natively. From a user's perspective, there is no difference between the two file formats, except that you obviously cannot use Marked to preview an OPML file (at least not yet...)
This means that you can use MMD Composer to work on your document, save it, and then reopen in OmniOutliner, for example, and drag and drop to rearrange the structure as needed. Save, and you can reopen right back in MMD Composer to get back to actually writing.
Alternatively, you can use Dropbox synchronization to work on an OPML file that is also used as a Mind Map in iThoughts on your iPhone or iPad. After diagramming your outline, the same OPML file can be opened in MMD Composer and you can get started on writing your masterpiece.
For more information on using OPML and MultiMarkdown, use the search box at http://fletcherpenney.net to search for "OPML workflow".