The origin of the term "deadline" comes from prison useage: A line beyond which the guards had leeway to shoot a prisoner who disregarded it.
I became familiar with the term through newspapers: This was a time beyond which copy submitted was too late to make the next edition of the paper. Once the type was set and the presses running, stopping them and re-setting type was a spendy and time-consuming process, and the only thing that would do that was a major breaking story that could not wait. WAR! or LOHAN ARRESTED ... !
This has become the more common meaning: A deadline is a time beyond which a project is late, and sometimes, irreparably so.
Deadlines in daily publishing or TV programming are necessarily tight, and missing one is a big no-no. You don't open the morning paper and find a block of white space in the middle of a page with a note that says, "Sorry, the story that was supposed to be here was late." Same thing with magazines or the evening news.
If you are writing a movie-tie in novelization and it needs to be on the racks the week the movie comes out, it has to be going to print X-number of weeks before that, and must be edited Y-number of weeks before that. The margin is narrower and with less give.
Sometimes, as in the case of an original book you are writing, these deadlines have wiggle room, they are considered "soft." Most writers I known, myself included, have had, if they work in a shared-world series, an experience wherein an editor absolutely-positively-has-to-have-it-by-the-1st-or-the-world-will-grind-to-a-halt! Writers will crank like they are on crank and make it under the wire.
Six weeks later, when they haven't heard anything, a follow-up note gets this response: Oh, um, I haven't had time to read it yet, sorry..
That editor's credibility goes into the toilet. Tell that story at a gathering of writers and watch the nods.
So if you have been around a while, you ask, when do you really have to have it? to see how much of a buffer the editor is giving him- or herself. On hurry-up projects, sometimes that buffer becomes critical.
But soft or not, eventually, there comes a point beyond which the piece is gonna be shot, and you as a worker do not want to cross that line.
I used to know an artist who was a pretty good illustrator. He had drawing chops, a good eye for detail, and his work graced several magazines for which I wrote, and some book covers in the F&SF field. He eventually faded from view, however, and it was not because he couldn't produce good art, it was because he couldn't get it in on time.
He always had a reason and it was never his fault.
There are legitimate excuses, of course. If your mother dies, your spouse gets deathly ill, you break your arm, you likely get a pass. And if you are terribly unlucky and two or three of these things happen to you in succession, editors will sometimes still cut you slack, as much as they can. Depends on how established you are, how good at it, how much they like you. But eventually it comes down to reliability. If you can't get the work done and in on time, they will look elsewhere, because there are people who can deliver. "Why" doesn't matter as much as "Doesn't."
I've been lucky in that I've made most of mine over the years, the deadlines. And it's now and again tricky.
If you have a collaborator, you are waiting on their draft and they are late? Not your fault, except that it is, because you are a team, and if team loses, you lose, and too bad. You can parse the blame among the players, but the final score is what matters. And mostly my collaborators have been conscientious and held up their end. (Not always. There have been some failures wherein I had to scramble and pick up more slack than I wanted, and that's disappointing, but part of the risk. What if your collaborator gets hit by a bus halfway through his or her draft? You have to be ready.)
I point all this out for those of you who are considering going into the shared-universes as I have done. If you get there, you will be better served by figuring out what the deadlines are, how flexible or inflexible they are, and making sure you observe the placement of your feet around them. You might not get shot, but if you miss deadlines too often, you likely won't get more work in that realm. You need to know this.