It's an unhealthy thinking pattern where you fixate on one small problem, or potential problem, and build it up in your mind until it is (or will be) the worst thing to ever happen in the history of humankind. Say you find one typo in an assignment after you've handed it in: rather than just kick yourself for it and then let it go, you stew over it until you're convinced the whole assignment is a write-off, you'll fail the subject, get kicked out of school and end up unemployable and homeless. All because of one typo in one essay.
It's quite an easy rut to fall into, especially if you tend to ruminate and obsess over things as a lot of Aspies do.
|Never trouble trouble, til trouble troubles you|
But worrying about the future won't change it. All it will do is ruin the present with constant stress and distraction, and burn up precious energy that could be used more productively. Many of the troublesome things we think we see through our precognitive long-distance goggles will never come to pass, and even if they do they probably won't be the utter disaster you're imagining.
Catastrophising can affect your relationships and your actions, and even bring the very thing you're worried about into being:
Say your catastrophising mind decides your partner is going to leave you because you think you saw her smile at someone else in a cafe, you can become so convinced you start snapping at her and being mean, treating her as though her bit on the side was already a confirmed fact. Meanwhile the partner, who just smiled because she was thinking about a nifty old TV show she found last night, is wondering why you're suddenly acting like such a complete knob... and whether you're worth the effort.
Having someone you can use as a reality check is really helpful. Being able to ask a trusted friend, family member, or someone in the same situation (like a fellow student who did the same assignment) can help slay the what-if demons and call a halt to the imaginary battles. It can be a valuable perspective recalibration to hear "Dude, even if you do get marked down, that essay's only worth ten percent of the grade for one subject. Get over yourself."
Another thing to bear in mind is that even if the worst does happen, the worst probably isn't that bad. So your partner leaves you: it'll break your heart, but ultimately you'll get through it and live to love again. So you get a bad mark for one essay: it'll sting enough to remind you to proofread the rest more carefully and maybe you'll have to work a bit harder on the other assignments for that subject to keep up your average.
But the world will not end.
At least, that's what I'm telling myself...