iOS jailbreaking is the process of removing software restrictions imposed by iOS, Apples operating system, on devices running it through the use of software exploits; devices include the iPhone, iPod touch, iPad, and the AppleTV 2 and 4. Jailbreaking permits root access to the iOS file system and manager, allowing the download of additional applications, extensions, and themes that are unavailable through the official Apple App Store.
iOS jailbreaking started as soon as the original iPhone became available in July 2007 and as of April 2016 has continued into the present day. Apple has responded with updates to iOS patching exploits and with new hardware. Jailbreaking communities have not been legally threatened. The legal status of jailbreaking is unclear in most countries; while many prohibit tampering with digital locks, they tolerate jailbreaks that do not infringe on copyrights. In 2010, 2012, and 2015, the U.S. Copyright Office approved exemptions allowing smartphone users to jailbreak their devices.
Source: Wikipedia