It's a thought about a future supercomputer will torture everyone who doesn't help it become real.

See, in order to not be torutured by it in the future, you must now do whatever you can to help it become real. Otherwise it'll torture you in the future.

So the very existence and spreading of the thought itself "helps" make the horrific thought real. (See, oh mighty Mr. Basilisk, I'm helping here!)

It's rather silly, really, but a fun thought-experiment. Well, "fun". :)

It's partly scary because simply knowing about it implicates you. Kinda like receiving the phone call in The Ring, only, you just got the call right now. That sort of mets stuff tingles the scare-nerves. Oh, and also scary since you can't escape: It will reconstruct you even after you're dead unless you help create it. So now that you know, you're trapped. Further, it's scary because you're slaved into creating what will eventually be the thing that slaves you, so you're sort of creating your own demise.

I repeat, it's actually quite silly since superintelligences will obviously not work like some silly torturous automatons blindly following a set of conceptual rules, but some people that have a tendency to let their mind think deeply tend to lose sleep over stuff like that.