Metaphor is a figure of speech which makes an implicit, implied or hidden comparison between two things or objects that are poles apart from each other but have some characteristics common between them. In other words, a resemblance of two contradictory or different objects is made based on a single or some common characteristics.

• My brother was boiling mad. (This implies he was too angry.)
• The assignment was a breeze. (This implies that the assignment was not difficult.)
• It is going to be clear skies from now on. (This implies that clear skies are not a threat and life is going to be without hardships).

http://literarydevices.net/metaphor/