Many consider metaphor to be at the heart of poetry (or even to define in part what it means to be human): the figure of speech that links dissimilar objects for their resemblance. For example, Emily Dickinson uses "the white assassin" as a metaphor for frost. Ground may have a blanket of snow where blanket is a metaphor for cover.
As a subject, metaphor is as complex and deep as one likes. Those interested in further exploration might cosider Julian Jaynes, "The Origins of Consciousness and the Bicameral Mind."