system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How does a jail broken phone work? Do they create a whole new OS or just add stuff on to the old OS?
gpt
Jailbreaking (Or Rooting on Android), is the same operating system it was before, you're just modifying the system to allow you to do things that were previously not allowed.  It's called jailbreaking because normally apps run in a jail or a sandbox that keeps them contained and limited.  So once you've modified the system you've literally jailbroken it.

iOS (and to a lesser extent Android), are designed so that everything runs in a sandbox that can't do any harm to the more important background stuff.  This is to prevent viruses and other malicious code from running and causing problems.  That sandbox is very restrictive though and prevents you from doing a lot.  Android lets you access a lot more, but until you root it, it's limited too.  (Like how you can't delete some default apps etc).  

Apple is slowing allowing more and more (custom keyboards, extensions etc), so jailbreaking is much less necessary that it used to be.  There are some things we'll never get without one though, (Caller ID spoofing apps and other 'kind of illegal' apps.)