This is a nice concise definition of the way metaphors work:
Metaphor is considered not so much as a way in which people speak but rather as a way in which people think. We use metaphors in our language because, to a large extent, we think metaphorically. The essence of metaphor, according to cognitive linguistics, is that we make use of our knowledge of one conceptual domain (the source) in order to gain new understanding of a second, non-related domaim (the target).
Pierre van Hecke, "Conceptual Blending" in Metaphor in the Hebrew Bible, pp 218-19
cited in Dearman, Hosea, NICOT, p11