To "Jailbreak" a device is a general term for overwriting the operating system on a device. The other terms are "unlocking" or "wiping". This will allow you to use the device on another networks provider. Lately the term has been associated with unlocking a iPhone. People used the term jailbreak because Apple did everything it could to prevent the source code from being overwritten because of AT&T's exclusivity to the service at the time. There are many helpful websites to assist you with unlocking an iPhone. there is only one issue. Many of the programs used can be "harmful" if done incorrectly. Or once done it's impossible to correct and your phone will be unlockable again.