Apple released iOS 12.4.1 along with tvOS 12.4.1 and watchOS 5.3.1 to address a serious vulnerability that was caused by Apple itself. The iPhone maker had it closed in iOS 12.3 but accidentally introduced the leak in iOS 12.4 again.
iOS 12.4 was supposed to be initially the last iOS 12 update ever since the new iOS 13 is coming within a few weeks. Apple was forced to release an extra update due to some security vulnerabilities. It is clear that jailbreaker pwn20wnd saw a flaw and released a jailbreak for iOS 12.4.
This is also the last update for the iPhone 6, iPad mini 3 and iPad mini 2. These devices will no longer be updated to iOS 13, which means that you will not receive any security updates in September. Also, more and more apps will stop working because an older iOS version is no longer supported.
The iOS 12.4.1 update is available via OTA on all compatible devices: just go to Settings-> General-> Software update to download this update for free.