Twenty years or so ago, Eliezer Yudkowsky, a former proto-accelerationalist, realized that superintelligence was probably coming, was
deeply unsafe, and that we should do something about that. Because he had a very hard time convincing people of this to him obvious fact, he first wrote a very good blog about human reason, philosophy and AI, in order to fix whatever was going wrong in people's heads that caused them to not understand that superintelligence was coming and so on. The group of people who read, commented on and contributed to this blog are called the rationalists.
(You're hearing about them now because these days it looks a lot more plausible than in 2007 that Eliezer was right about superintelligence, so the group of people who've beat the drum about this for over a decade now form the natural nexus around which the current iteration of project "we should do something about unsafe superintelligence" is congealing.)