Cydia is a Debian APT package manager that lets you download tweaks and apps that haven't been approved by Apple for your jailbroken iPhone, iPad and iPod Touch, via repositories.
Repositories with additional packages can be added to Cydia through "Manage->Sources->Edit->Add".
Cydia is preinstalled with most iOS jailbreaks, since iPhone OS 2.0.
Cydia for iOS 11, updated by Saurik, is officially supported in unc0ver by @Pwn20wnd. (It is also compatible for iOS 12)
In December 2018, the Cydia Store has been shut down, which only affects 3 of the main default repos, including BigBoss. Because of this, one is no longer able to buy tweaks from those 3 repos, but one can still download the free ones.