The iOS 12 jailbreak saga all started when Brandon Azad, a Google Project Zero member, released his voucher_swap exploit that grants the user tfp0 on iOS 11/12.
Brandon Azadís exploit set off work on a jailbreak for iOS 12. This jailbreak will undoubtedly be quite important as many jailbreakers are looking to upgrade from a newer version of iOS after having skipped iOS 11 due to its performance issues

Getting tfp0 (task_for_pid(0)) is essentially the same as being able to execute code with kernel privileges which is the highest privilege that one can run code with. As of right now, the exploit only works on devices with a 16K page size (Apple A9 devices and newer) but Pwn20wnd is working on getting it to function on older devices (Apple A7/A8) that have a 4K page size.
With this exploit, work on an iOS 12 jailbreak can start being made although there are still various things that need to be figured out like a CoreTrust bypass in order for such a jailbreak to be created. However, users on iOS 11 have already started benefiting from this exploit as unc0ver and Electra (main iOS 11 jailbreaks) have been updated to work with this exploit which brings about a better success rate AND iOS 11.4/11.4.1 support!
Note about tihmstarís exploits: Shortly before Brandon Azad released his exploit, tihmstar released treadm1ll which is a tfp0 exploit for iOS 11.4/11.4.1. This was released without the cleanup code and as a result, it doesnít work in its current state. Then, earlier today, tihmstar released an exploit called v1ntex which is based on the same bug as voucher_swap (CVE-2019-6225) but apparently, this is not working out of the box either. However, this exploit has support for 4K devices so perhaps Apple A7/A8 devices will have tfp0 on iOS 12 soon!