Ernest Hemingway had a distinctive style of minimalist writing resembling an iceberg: most of the story existed beneath the surface. What you saw was only a fraction of the sum total. Hemingway went so far as to claim that when you omitted something, the redaction strengthened your story. Much like the part of an iceberg beneath the waves, unmentioned backstory lends a certain gravitas to the visible portion.
So we can make our stories better by leaving things out. Great. But which parts do we leave out? Randomly removing sentences is unlikely to impress anyone. After a few moments of thought, you might conclude that you should cut out repetitive language and unnecessary side plots. While probably a good move as far as general editing is concerned, that’s not what the Iceberg Theory is about.
Instead, when you read over your prose, look for places where you connect all the dots for your reader. Then hit the delete key. If you have some chemistry brewing between your characters, remove all overt references to emotion so your readers hang on every line of dialog trying to figure out just what is going on between those two. If one of your characters comes to some profound epiphany at the climax of your story, remove its explanation from the prose.
The idea is that you as the author know exactly what is going on, but you don’t give your reader the same benefit. Instead, you force them to dig for the deeper meaning in your writing. Why? Because despite what anyone might say to the contrary, readers enjoy a little ambiguity. They want to read things that make them think. So when you’re laying down the dots for your audience to connect, try leaving out a few to see if things resonate better. You can always add the words back if they’re needed.