This is iceberg as in the force that sank the Titanic, not the lettuce!
Talking to a student the other day about her (very good) work, I used the old iceberg analogy. This is where we say that a writer should only show the reader a small percentage of all that research and thinking that we do to get to the final product. The rest is sitting under the water, not visible but certainly holding up the work that the reader is enjoying. My student felt the desire to include all that she knew about the topic – and she knew a lot. I was asking her to cut some of that information out and trust that her writing would resonate with all that she knew, even though it might not all be specifically mentioned.
A good exercise for practising this is to take a scene that you’ve already written and rewrite it completely. This time, you have to leave out a key part of the scene or some key events (that’s the ice under the water) and make the scene you write (the visible ice) convey that part of the story through feeling or in some other way.
Why not try it?