It is the resource that upgrades Legendaries from 265ish to 290ish.
You get it from ZM activities, like elites and chests and events and the weekly status bar thingy. Torghast also gives you it. You also get it from Raiding and Bgs. My Rogue has tons just from Random BGs. I did almost no Torghast this Patch. Most of mine has come from Raiding.
People are discussing it a lot lately, because they are adding a way to move Cosmic Flux between Characters = send it to Alts. A bit long over due, but it is on the way.
The way I heard about it was kind of funny - someone made a thread in GD requesting it, and a Blue responded lol
This is the way Blizz should communicate, hehe.