What is a simultaneous submission?

Recently, I submitted a short story for possible publication and in reading the guidelines I noted the typical statement of "Please no simultaneous submissions."  Then, even though I knew what it means, possibly others are asking the same question: what is a simultaneous submission?

In simple terms, for publications, simultaneous submissions means you submit one piece of your work to one publication at a time.  That's it.  Nothing complicated about it.

It could mean that even though you've submitted your work to one publication (or contest), it could be six months or more to hear back from them.  That's okay.  Just keep writing new pieces and submitting them other places.

The reason publications (or writing contests) do this is because of the following scenario: let's say you submitted it to two publications, then both accept it.  Now you have the painful process of advising one of them that they can't have it.  The result could be that both publications, even though they specified no simultaneous submissions, will not publish your work.

And that's not a very good place to be.