The Oxford Dictionary definition of a metaphor is the
"application of a name or descriptive term or phrase to an object or action where it is not literally applicable".
In this way, the metaphor is itself a semiotic sign providing meaning obtained from what is referenced. More than that, the metaphor adds to other communication by providing the association of a meaning to an unfamiliar object rather than only a familiar one. This extra ability of metaphor adds an extra layer to sign-object meaning model. Extending from the word metaphor, when we introduce the concept of a "family tree" to someone for the first time, we allow the pre-existing identity of a tree to help provide meaning to the way we can look at family history. The metaphor acts as a super-interpretant, providing an interpretation where none would normally exist. This super-interpretation makes the metaphor a vital tool in computer interface development as it can be used as a powerful sign for the audience viewing it.