Last week, it was reported that there was a new public jailbreak for iPhones running iOS 12.4, the first of its kind in a few years. Just as you'd expect, Apple was quick to release a patch for the vulnerability, releasing iOS 12.4.1, a supplemental macOS 10.14.6 update, tvOS 12.4.1, and watchOS 5.3.6.
In the official changelogs, they all say the same thing except for the watchOS one, for which there is no changelog. The rest of the list the one single vulnerability, crediting security researcher Ned Williamson of Google's Project Zero.
Interestingly, Apple also gave credit to @Pwn20wnd for their assistance with the matter, presumably just for finding the exploit.
While jailbreaking iPhones is a thing that some people do, this is an update that you're going to want to install on your devices sooner rather than later. After all, it is a security patch.