Short fiction is like low relief. And if your story has no humor in it, then you're trying to look at something in the pitch dark. With the light of humor, it throws what you're writing into relief so that you can actually see it.

This quote is saying that short fiction is like a sculpture that is slightly raised or raised in some parts, rather than being completely flat. The idea is that humor acts like a light in a dark room, making the sculpture easier to see the details. In other words, humor can help the reader to better understand the story and to be able to “see” the details more clearly.