When an existing standard becomes almost impossible to supersede because of the cost or logistical difficulties involved in convincing all its users to switch something different and, typically, incompatible.
Things which have been accused of benefiting from lock-in in the absence of being truly worthwhile include: the QWERTY keyboard; any well-known operating system or programming language you don't like (e.g., see "Unix conspiracy"); every product ever made by Microsoft Corporation; and most currently deployed formats for transmitting or storing data of any kind (especially the Internet Protocol, 7-bit (or even 8-bit) character sets, analog video or audio broadcast formats and nearly any file format).
Because of network effects outside of just computer networks, Real World examples of lock-in include the current spelling conventions for writing English (or French, Japanese, Hebrew, Arabic, etc.); the design of American money; the imperial (feet, inches, ounces, etc.) system of measurement; and the various and anachronistic aspects of the internal organisation of any government (e.g., the American Electoral College).