Jailbreaking an iPhone involves altering the phone's operating system to remove restrictions on the hardware and software of the phone. The primary benefit of jailbreaking is the ability to use software that is not approved by Apple. However, jailbreaking may cause problems with the normal operation of the phone.
Software that runs on a normal iPhone must be authorized by Apple through a digital signature process that prevents unauthorized applications from running. While this is partially a security feature, it also prevents software with nonstandard features that may be useful from running. A common example of this type of program are Web browsers that allow direct downloading of files to the phone, which is not typically supported by Apple's Safari and other approved browsers. Jailbreaking a iPhone involves bypassing the digital signature requirements in the iOS operating system so that programs without digital signatures can run on the device.
Jailbreaking an iPhone is not without risks. Jailbroken iPhones sometimes suffer from stability issues because not all unsigned software is stable. The alterations to iOS performed as part of the jailbreaking process may also contribute to crashes and program instability. It is also potentially easier for a jailbroken iPhone to be infected with malicious code, and it is more difficult to back up a jailbroken iPhone or update the operating system.