"Simultaneous submissions" is one of them. Not to be confused with "multiple submissions."
When a magazine discourages "multiple submissions", that means that the editorial staff don't want you submitting all thirty of your short stories to them at once. Even if they accept multiple submissions, it's still not a good idea to bombard them like that. Be humane.
Simultaneous submissions, though, means something else entirely. If a journal requests "no simultaneous submissions," that means if you submit one story for a publication's consideration, you cannot submit it to any other publication until they decide upon the story's fate.
When starting out in this biz, a first compulsion might be to just formulate a list of attractive publications and send them all your stuff... all at once. This is a terrible, terrible idea, and a sure way to burn bridges. It's best to research each publication on an individual basis and formulate the appropriate strategy..