That's something I'm clueless about. What on earth are they?
They are mushrooms, that grow on the earth.
People eat them, and they're slowly poisoned, because they contain hydrazine, a type of rocket fuel. Didn't you click the link I provided?
Beefsteak mushrooms are a good metaphor for UFOlogy. Seems tasty at first, and the poison takes effect so slowly, that you never even notice it. But, it's cumulative, and devastating. The rocket fuel aspect is just a bit of divine irony, IMO.