Jailbreaking is the process of removing software restrictions from your device via exploits. It pretty much allows you to install custom themes, easily sideload apps, install useful modifications and tweaks, and on older devices, fix services that are no longer working.
There many different methods of jailbreaking, some easier than others.
Requires your device to have working Power and Home buttons in order to put your device in DFU mode. This jailbreak method also depends on the hardware and OS of the computer you are jailbreaking with, and what type of device you have.
Wipes all data on your device, while jailbreaking it in the process. This is performed automatically on downgrades.
Jailbreaking a device heavily depends on what iOS version it's running, and which device it is.
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