Most Jailbreaks today are "tethered" in some way, which means the Jailbreak disappears (to varying degrees†) once the phone is turned off. For Jailbreaks like unc0ver, this means you need to re-run a bootstrap app every time you reboot your phone, in order to return to "Jailbroken" mode and allow unsigned code.

This, of course, is a catch-22. You need to run an app to allow unsigned apps, but that app can't run if it isn't signed.

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† The community makes a distinction between "tethered", "semi-tethered", and "semi-untethered" jailbreaks. The jailbreak I described above is "semi-untethered". You really couldn't come up with terminology more prone to getting mixed up...