As the name suggests multiple submissions refers to when a writer sends out the same piece of writing to more than one publisher at a time. An action that for obvious reasons is, on the whole, approved of by writers and disapproved of by publishers.
As a writer it can be frustrating to wait several months for a publishing house to get back to us and say whether they like our work or not. Let’s face it, unless we send out multiple submissions it can take a year to approach just four or five publishers with our work. What if it takes submitting to ten publishers before it’s accepted - that’s two years or more.
Publishers, understandably, on the other hand, don’t want to waste their time reading and thinking about a submission that could already have been snapped up by another editor.
Over the years I have always stuck with the principle of never sending out submissions to more than three publishers at once. Not that everything I write is suitable for multiple submission.