It's the wildfire. Cersei's going to go nuts and blow King's Landing up so that Cleganebowl can't happen.

It's been strongly hinted at in the books that Cersei will burn down Kings' Landing. There's a lot of parallels between her and Aerys II Targaryen.

did they use up the wildfire against stannis in the naval invasion? or was that new stock?

either way I doubt cersei would, losing Kings landing means they will likely lose the crown, then they're worth nothing

Aerys (The Mad King in power before Robert's Rebellion) stockpiled wildfire all around King's Landing during the war (Beneath the Red Keep, the Great Sept of Baelor, the Dragonpit and other major thoroughfares) with the intention of burning king's landing to the ground in the event of Robert's army reaching the city. This was why Jaime ended up killing the king, and all the Grand Pyromancers - to stop this happening after Pycelle had persuaded Aerys to let the Lannister host inside the city walls.

Thing is, killing all the Pyromancers saved the city that day but also meant that all the people who knew where the wildfire was were dead, and in th years since caches of the stuff have been found all over the place and kept a lot of fear about wildfire in the air.

The stuff that was used against Stannis in the War of Five Kings was comissioned by Cersei for defence of the city, then later procured by Tyrion in his actual strategy. The stuff Cersei used to burn down the Tower of the Hand (In the books) and will likely use to burn King's Landing has been hidden there for decades.

The city burning down would coincide with Daenerys' visions from S2 of walking through the Iron Throne room and seeing everything destroyed and winter setting in.

Cersei probably would use it at this point. all she cares about is her family (aka Tommen and Jaime) and both are slowly being pulled away from her. She's already shown willingness to use wildfire on the Tower of the Hand because that's where Tywin died, and she's generally showing a lot of the same signs as Aerys did before he died. She would quite readily martydom the entire realm as an act of madness at this point if she loses Cleganebowl the trial.

Considering that the show has been dropping hints about hidden Wildfire caches and quotes about cities being burned, I think it's safe to assume the rumor definitely isn't about the Wildfire.