Submitting work without any prior contact or particular solicitation in the hope that someone will take notice and publish it goes by many names--"over the transom" submission and "blind" submission, for example--and once on the editor's desk constituting what is often referred to as the "slush pile."  None of these terms sounds promising. 
This is still, nevertheless, a time-honored and reasonably successful tradition.

  • ADVICE. You can and better make your own luck, however.  Sending your science fiction manuscript blindly to a publisher of nature guides is obviously a bad move.  But be sure you understand the humor here.  If you are taking care of your own work, you must know your field as well as you can.  Be certain to read publisher descriptions ahead of time.  Spend quality time in the library, and take good notes.  Get advice everywhere you can.
  • We don't see transoms on doors much anymore--which we may take to be symbolic.  Transoms were windows above doors, which could be opened for ventilation.  All the old detective movies had transoms in them, as they were good plot devices: If you stood on a chair, you could look into a room.  Keyholes used to work that way, too.  Of course, the door remained closed in these scenes--that was the point, and still is.  The idea here is that the transom represented a chance, or an edge, a possibility.  Curiously, transom is also the name of the horizontal beam on a cross or gallows, so that edge and possibility become double-edged swords.