"So, what's a tethered jailbreak?
A tethered jailbreak is a jailbreak which leaves the jailbroken phone unable to boot without the help of a computer. Any time such a phone is completely powered off, restarted (for instance after installing certain jailbreak apps) or its battery dies, it must be plugged into a computer and the jailbreaking application must be re-run to get it past the Apple logo boot screen. Such a phone does not lose its jailbreak, and all customizations will be preserved, but the phone will not be able to boot past the Apple logo screen on its own."