An Interface metaphor is a set of user interface visuals, actions and procedures that exploit specific knowledge that users already have of other domains. The purpose of the interface metaphor is to give the user instantaneous knowledge about how to interact with the user interface.
The folders and the file cabinet representation of the file system of an operating system is an example of Interface metaphor. Another example is the tree view representation of a file system, as in Windows Explorer, that helps a user to intuitively use it.
Reference:
http://en.wikipedia.org/wiki/Interface_metaphor
