Jailbreaking was a popular community effort to unlock the iPhones, especially in the early days of smartphones. Nowadays jailbreaking has become harder as Apple's continuous patches break the exploits.
This one is said to be unpatchable due to the fact that the affected part of the system is the boot ROM which means that it is a read-only memory and thus can't be patched with a simple software update.
Unfortunately for the jailbreak community the exploit does not cover the most recent phones, including last year's iPhone XS and this year's iPhone 11 models. However, this might make iPhone X and earlier iPhone models more attractive choice for users that appreciate homebrew and tinkering with their technology.
Finding the exploit is only the first step. Now the community has to figure out how to create a jailbreak to unlock each affected iPhone model. Even though this exploit could potentially allow a jailbreak, it is not necessarily possible.
Exploit also opens up older iPhones for more nefarious hacks. This might allow easier access to data from stolen phones or perhaps malware to be installed on second-hand devices.
EPIC JAILBREAK: Introducing checkm8 (read "checkmate"), a permanent unpatchable bootrom exploit for hundreds of millions of iOS devices.-- axi0mX (@axi0mX) September 27, 2019
Most generations of iPhones and iPads are vulnerable: from iPhone 4S (A5 chip) to iPhone 8 and iPhone X (A11 chip). https://t.co/dQJtXb78sG