The Cydia app store has been the go to location for users who have jailbroken their iPhone to get apps.
The store provided access to apps that Apple blocked from their app store. It also provided a way to gain access to paid apps for free.
The owners of the site shut it down due to costs and security issues. The store was losing them money to run.
There are other app stores for jailbroken iPhones but the Cydia app store was by far the largest and most popular.
It is less popular to jailbreak iPhones in recent years due to Apple making it more difficult with quicker security updates. Many of the features users were jailbreaking their iPhones for have also been added to iOS.