The meaning of allegory

From Slate: "An allegory is not just another word for a metaphor. In essence, it's a form of fiction that represents immaterial things as images. … The distinction might seem obscure and academic to many. Shouldn't allegory be grateful to get any attention at all? Isn't it just an archaic literary mode that nobody uses anymore? Yes and no. About the only people creating true allegories today are political cartoonists. But a culture never entirely discards its roots, and allegory, which first appeared in the waning years of the Roman Empire, is one of the foundations of Western literature."