"Iceberg theory" originated from Hemingway's belief that if the writer knows what he is writing and writes truly enough, the readers will understand the story the same even when the writer omits some parts.
In A Farewell to Arms, Hemingway writes about the single life of Lieutenant Henry in World War I, although there were thousands more soldiers who attended the war. Here Henry represents the tip of the iceberg; Hemingway is trying to convey the lives of many soldiers through the story of just one.
In another perspective Hemingway describes Henry's emotions as confined as possible but in reality there is a whole iceberg of emotions underneath the surface. When Cathrine dies after the child birth Hemingway says “It seems she had one hemorrhage after another... I went into the room and stayed with Catherine until she died.” Hemingway does not explicitly describe Henry's grief here but readers can still sense the sorrow beneath the surface.