The Iceberg Theory, also called the omission theory, is where the true meaning of a piece lies beneath the surface. What you see on the surface is not as important as what lies deeper. So, for example, in Hemmingway's stories, quite often he will focus on the surface elements as a 'tool' for allowing the true theme of the piece to shine through from below.
In Elegance of a Hedgehog, by Murial Burbury, it is evident in Renee's insistance on adhering to the stereotypes regarding women, and particularly women of her age. The surface character is NOTHING compared to who she really is.