“iOS jailbreaking is a process that allows devices running Apple’s iOS (also known as iPhone OS prior to iOS 4.0) operating system (such as the iPad, iPhone, iPod Touch, and recently Apple TV) to gain full access (root access) to unlock all features of the said operating system, thereby removing limitations imposed by Apple. Once jailbroken, iOS users are able to download additional applications, extensions and themes that are unavailable through the official Apple App Store, via installers such as Cydia, one of a number of current means for older iPhones. A jailbroken iPad, iPhone or iPod Touch running iOS can still use the App Store and iTunes and other normal functions, such as making telephone calls. Jailbreaking is similar to rooting an Android device.
At its core, jailbreaking a device gives access to its root filesystem, allowing modification and installing Third-party software components. This gives the user more control over the device and may enable features that were previously unavailable. In many cases, jailbreaking also voids the device’s warranty.” Source: Wikipedia.