Even for the most hardcore of Game of Thrones fans, sometimes it's hard to keep track of the millions of characters and places in Westeros. So if you were watching Game of Thrones Season 7, Episode 4, "The Spoils of War," you might have noticed all this talk of the Iron Bank between Cersei and Tycho Nestoris (aka Mycroft from BBC's Sherlock.) Well, here's why the Iron Bank on Game of Thrones matters so much.
So essentially, the Iron Bank of Braavos is the biggest, most powerful bank in the entire world of Game of Thrones. The Iron Bank is located in the free city of Braavos, and has clients all across Essos and Westeros. You all know how the Lannisters always pay their debts... well, the Iron Bank will always remind its clients to pay the heck up when it comes to loans. ("The Iron Bank will have its due.")
The reason why we care so much about the Iron Bank in Season 7 is because the Crown of Westeros (aka Cersei Lannister) owes the banks some major cash. The bank sends Tycho Nestoris — played by Mark Gatiss, who you may recognize as Mycroft Holmes from BBC's Sherlock — to King's Landing to collect this debt. We also learn that the bank is definitely Team Lannister, because of how much money it's made from the slave trade — an institution that Daenerys Targaryen destroyed.
In Episode 3, Cersei convinces Tycho to wait in King's Landing, promising that her brother Jamie will be back with the gold she owes soon. Unfortunately (spoiler!), Jamie and his army have a run in with Khaleesi and her dragon, and it seems unlikely he'll get that gold back to King's Landing after all.
This is potentially worrying for Cersei, because she really needs that support from the Iron Bank if she has any hope of beating Daenerys. It looks like the Iron Bank has once again backed the wrong client. (RIP, Stannis.)