A metaphor: a figure of speech in which a word or phrase literally denoting one kind of object or idea is used in place of another to suggest a likeness or analogy between them (as in drowning in money); broadly : figurative language
For instance, when someone says something like "call off the dogs", it does not mean people are dogs. It means to have those people stop hunting or searching for something or someone like search dogs.
It is funny because I heard from a friend that someone they knew with a lot of education took it literal. I hope the voucher people don't hear about this, they would want to privatize every school in Missouri. :)