Orphaned attachments are file which were uploaded via "attachments" function during creation of a post of pm.
But that post or pm was never submitted, maybe because the user only used "Preview" instead of "Submit", closed his browser window, had problems with network connections ...
So, the file was uploaded with success and a entry in the corresponding attachment table was created, but no entry in the post or pm table.
There is a section in the acp called (Whow - what a surprise!) "Orphaned attachments" - from there you can view the attachment by a click on the file name.
Normally you can delete those entries - the delete function will delete the attachment file and the database entry.