What is Hemingway trying to say in his iceberg theory?

Ernest Hemingway's "Iceberg Theory" deals with the basic principle that "less is more." Instead of stating the obvious, Hemingway attempts to use dialog and subtext to convey his themes. In revision, cutting becomes more important than adding material. Needless repetition and irrelevant information should be avoided. Hemingway likens this style to an iceberg since only a fraction of it lies visible above water; the rest--the greater mass--is unseen below. A savvy reader will uncover the missing parts if the story's message is delivered in a short but succinct manner.

If a writer of a prose knows enough about what he is writing about he may omit things that he knows and the reader, if the writer is writing truly enough, will have a feeling of those things as strongly as though the writer had stated them. The dignity of movement of the iceberg is due to only one-eighth of it being above water. The writer who omits things because he does not know them only makes hollow places in his writing.  --Hemingway, Death in the Afternoon