Jailbreak means to unlock the operating system of a mobile phone or other device so that it can run software which it is not normally authorized to use because of restrictions imposed by the device's manufacturer.
If someone jailbreaks a smartphone, tablet (computer) or other device, they use a special piece of software in order to remove the restrictions imposed by the manufacturer on the kind of applications that can be downloaded and run on the device. The concept of jailbreaking is most often associated with products from Apple Inc., such as the iPhone, iPod and iPad, which routinely restrict the user to applications licensed exclusively by Apple and purchased via its App Store. As well as wanting to lift the restriction on what kinds of applications they can purchase, those who jailbreak often do so as a reaction against what they perceive as a form of censorship imposed by Apple Inc. in only allowing the use of their 'approved' apps.
The concept of jailbreaking dates back to July 2007, when it was applied to the iPhone within a month of the device's first release. This first occurrence related to the adding of custom ringtones, but the idea quickly developed as a means to augment the phone with games and other applications not licensed by Apple Inc.