Metaphor

   A. Metaphor

Metaphor is a figure of speech which makes an implicit, implied or hidden comparison between two things that are unrelated but share some common characteristics. 
Common Speech Examples of Metaphors

• My brother was boiling mad. (This implies he was too angry.)
• The assignment was a breeze. (This implies that the assignment was not difficult.)

Literary Metaphor Examples
“She is all states, and all princes, I.”
John Donne, a metaphysical poet, was well-known for his abundant use of metaphors throughout his poetical works. In his well-known work “The Sun Rising,” the speaker scolds the sun for waking him and his beloved. Among the most evocative metaphors in literature, he explains “she is all states, and all princes, I.” This line demonstrates the speaker’s belief that he and his beloved are richer than all states, kingdoms, and rulers in the entire world because of the love that they share.

Functions
From the above arguments, explanations and examples, we can easily infer the function of metaphors; both in our daily lives and in a piece of literature. Using appropriate metaphors appeals directly to the senses of listeners or readers, sharpening their imaginations to comprehend what is being communicated to them. Moreover, it gives a life-like quality to our conversations and to the characters of the fiction or poetry.
References
http://literarydevices.net/metaphor/