A metaphor is using a name for something that is not literally applicable, to convey an idiomatic subtext or deeper meaning.
We call it the "eye of a potato," but it's not really an eye. A glaring error doesn't really glare, and the tongue of a shoe isn't really a tongue, it only reminds us of a tongue. These are metaphors, names for things that aren't actually what the things are. Poetry is often metaphor, and Slang is metaphor, like 'really cool'- well it's not really cool, in the temperature sense of the word, but we like it. Maybe it's just neat... (but not necessarily tidy). If it's "the bomb," or it's "bad," of course we know its the opposite.
The Tree of Life is a metaphorical construct, used to organize a myriad of sub-metaphors. If language represents the passage from Chochma to Binah, metaphor is at the frontier of language itself, bridging the literal from the barely imagined. Letters, the building blocks of language, appear in Key 0, while their combinations, words, appear in Key 1.
Dreams are metaphors, and parables are also metaphors. Using an imaginary story to illustrate deeper concepts, the parable teller imparts wisdom in ways that are easier to remember. Metaphors often stretch reality past the point of possibility, teasing our minds into new territory.