JailbreakJailbreaking is the process by which full execute and write access is obtained on all the partitions of iOS, iPadOS, tvOS and watchOS. It used to be done by patching /private/etc/fstab to mount the System partition as 'read-write'. This is entirely different from an unlock. Jailbreaking is the first action that must be taken before things like unofficial activation (hacktivation), and unofficial unlocking can be applied.Older jailbreaks also included jailbreak app modifying the AFC service (used by iTunes to access the filesystem) to give full filesystem access from root. This was later updated to create a new service (AFC2) that allows access to the full filesystem.Modern jailbreaks now include patching the kernel to get around code signing and other restrictions. These are called Kernel Patches.Note: The legality of jailbreaking your device varies with each country/region. Wikipedia has a summary of legality for some countries.

Types of JailbreaksWhen a device is booting, it loads Apple's own kernel initially, so a jailbroken device must be exploited and have the kernel patched each time it is booted up.An untethered jailbreak uses exploits that are powerful enough to allow the user to turn their device off and back on at will, with the device starting up completely, and the kernel will be patched without the help of a computer – in other words, it will be jailbroken even after each reboot.However, some jailbreaks are tethered. A tethered jailbreak is only able to temporarily jailbreak the device jailbreak ios 15.2 during a single boot. If the user turns the device off and then boots it back up without the help of a jailbreak tool, the device will no longer be running a patched kernel, and it may get stuck in a partially started state, such as Recovery Mode. In order for the device to start completely and with a patched kernel, it must be "re-jailbroken" with a computer (using the "boot tethered" feature of a tool) each time it is turned on. All changes to the files on the device (such as installed package files or edited system files) will persist between reboots, including changes that can only function if the device is jailbroken (such as installed package files).

In more detail: Each iOS device has a bootchain that tries to make sure only trusted/signed code is loaded. A device with a tethered jailbreak is able to boot up with the help of a jailbreaking tool because the tool executes jailbreak download exploits via USB that bypass parts of that "chain of trust", bootstrapping to a pwned (no signature check) iBSS, iBEC, or iBoot to finish the boot process.