Or rather, Random Number Generator. Basically it means the random things that happen in fight, like who gets unbound magic on Sindragosa (a debuff that pretty much disables casters on hardmode) or valkyrs (2 valkyrs that fly down and pick up players and throw them off the edge). So like on the Lich King encounter, you need two discipline priests to spam shields on the entire raid, to manage a dot that Lich King throws on the entire raid every now and then, called Infest. Now, if one (or both) of the priests get picked up by the valkyrs, you won't be able to shield the entire raid, which might lead to a wipe. Of course, you can protect yourself against this by bringing 3 disc priests, but that's not a very good healing setup then.
That's RNG for you. Sometimes it'll completely botch your attempts at an encounter, sometimes it'll help you. It doesn't decide the entire game of course, but sometimes it can easily steer you towards a wipe.
Hopefully I can pick it back up again soonish.
