Jailbreak is escaping from jail
However, that is the translation from English. Nowadays, jailbreaking means cracking your hardware so you can do more with it. Since the advent of the IPhone, jailbreaking has become popular. Normally, the hardware (IPhone) is equipped with an operating system with limited capabilities. In the case of the iPhone you can only install applications from the itunes Store, after jailbreaking you can also install applications that are not in the itunes store. Jailbreaking is a fairly harmless process: a number of operating system files are modified (and can be rolled back if desired). The biggest reason to jailbreak is because more is made possible. When the APP store offers enough software, it is not necessary for most people to jailbreak their phone.