Metaphors are a type of figure of speech where a different word, words, or a phrase is used instead of more specific, precise, or mundane phrasology for any given concept. When metaphor works it is because the replacement word or phrase says something new and broadens the menaing of the original concept. A metaphor's functionality also depends on it being at least partially transparent to many people. Metaphor brings a sort of comparison, By using one expression for the other the two things are compared or perhaps overlapped, gradating and shading meaning.
The word metaphor in English comes from the Greek roots μεταφορά (metaphorá) meaning to transfer, and from μεταφέρω (metaphero) meaning to carry over, to transfer and furthur from μετά (meta), meaning between and φέρω (phero), meaning to bear, to carry.
Metaphor aas a category also contains rhetorical figures of speech that achieve their effects via association, comparison, and resemblance, e.g. antithesis, hyperbole, metonymy, and simile; all are species of metaphor.
The best wordsmyths of the English language, be they poets, playwrights, authors, lyricwrights, speech writers, and/or extemporaneous speakers, often create new metaphors, metaphors that even though they have never occurred to one before, are immediately apparent and expand (or in some cases explode) the understanding of the original concept.