We've talked about it a few times now. Its the current plot advancement. There's going to be 4 books and the most recent was Broken Realms:Teclis, with the recent big Lumineth release being part of it. Each book is significantly moving the plot and world forward. I havent read BR: Teclis yet but BR: Morathi involved her accidentally freeing Slaanesh, mostly, eating a ton of elven souls, ascending to near godhood and capturing a major city of sigmar. Breaking the alliance between Order in the process. Thus significantly shaking up the world.
The most recent book from what I understand involves Teclis and the Lumineth invading the realm of the dead to beat up Nagash as punishment for breaking things all the time.
its actually really interesting to see how GW is happy to radically change the world and advance the plot in AoS, compared to the stagnant Old World and how they constantly retconned, ignored or flat out didn't do any plot advancement for years.