In Episode 8 of this season of Game of Thrones, King Tommen announced that there would be no more trials by combat, meaning Cersei could not use the newly enhanced Mountain to fight for her freedom. With no other options and a son she can no longer rely on, Cersei turned to her right-hand man, Qyburn, who tells her that his little birds investigated the rumor she told him about and they found that it's more than just a rumor. At this point everyone's pretty sure that the rumor was about the wildfire that's hidden at King's Landing but why is there wildfire hidden in King's Landing in the first place?
If you're a Game of Thrones fan, you've heard about the Mad King a few (100) times by now. He was Daenerys' father, King Aerys II, and he was, as his nickname indicates, a bit crazy. After his son, Prince Rhaegar, took Lyanna Stark, her brother, Brandon, protested the taking of his sister and was arrested for treason. His father, Rickard Stark, went to King's Landing to ransom him back, but Aerys had them both killed. This led to the houses of Stark, Tully, Arryn, and Baratheon to raise a rebellion, known as Robert's Rebellion.
During the rebellion, Aerys had his new Hand of the King, after Tywin Lannister betrayed him, hide catches of wildfire all over King's Landing. His plan was to burn down King's Landing, killing the half million inhabitants of the Westeros capital. Aerys believed this massive fire would transmute him into a dragon and he could then obliterate his enemies. This was when Jaime stepped in and killed Aerys, thereby becoming known as the King Slayer. Jamie never told anyone about Aerys' plans, except Brienne of Tarth, because he thought no one would believe him, and thus the wildfire remained a myth.
The last time we saw wildfire was in Season 2, when Tyrion loaded a ship with 8,000 jars of wildfire to meet Stannis' ships and then had Bronn hit it with an arrow, causing it to blow up. The wildfire was so strong the fire then burned other ships that weren't directly hit and Stannis' men were burned alive.
There's not doubt wildfire is very powerful. If Cersei really plans to use it on King's Landing, everyone better start running, because no one will be able to survive the blast.