A metaphor is figure of speech that draws an equivalence between two unrelated items. Unlike a simile, it does not use "like" or "as", and uses equivalence rather than a similarity.
House often uses metaphor to explain a concept in physiology or etiology to his fellows or, in less common cases, to his patients or supervisors. In the series, they serve the purpose of explaining very complicated concepts of medicine to the audience in plain language.
"Well come on, sit on grandpa's lap as I tell you how infections are criminals; immune system's the police"
"Cervical lymph node is a garbage dump. Very small one--just one truck comes, and it only comes from one home."
"Saying there appears to be some clotting is like saying there's a traffic jam ahead. Is it a ten-car pile up, or just a really slow bus in the center lane? And if it is a bus, is that bus thrombotic or embolic?"
―Euphoria, Part 1
"[The Inuit] look for the blue heron, because there's no way to see the fish. But if there's fish, there's gonna be birds fishing. Now, if he's got hairy-cell, what else are we gonna see circling overhead?"
"The tumor is Afghanistan, the clot is Buffalo. Does that need more explanation? OK, the tumor is Al-Qaeda. We went in and wiped it out, but it had already sent out a splinter cell--a small team of low-level terrorists quietly living in some suburb of Buffalo, waiting to kill us all. . . . It was an excellent metaphor. Angio her brain for this clot before it straps on an explosive vest."
"The liver is like a cruise ship taking in water. As it starts to sink, it sends out an SOS. Only instead of radio waves, it uses enzymes. The more enzymes in the blood, the worse the liver is. But once the ship has sunk, there's no more SOS. You think the liver's fine, but it's already at the bottom of the sea."
However, House's use of metaphor is not limited to medical matters. He often uses them to explain philosophical concepts as well.