It's for all Office miscellaneous stuff (as you now probably have figured
out...)
That said, I don't show the Document Properties pane when I save documents
in Office 2007 (on Vista), on a standalone PC with no server environment. If
I choose to launch it, then I can click the "x" in the upper right to launch
it. It's a manual process.
If you are using Sharepoint to save files to, I believe that document
properties are turned on by default. Is that what you're experiencing? When
you save, does it then pop up a required dialog? What operating system are
you using?