The Iceberg model is a term that likens events; personal, political, social, etc. to an iceberg where only an estimated 10% of the ice is above water while 90% of the iceberg is submerged.
For instance, this means that every story we see on the news is like the top 10% of the iceberg. These newsworthy events don't exist in a vacuum. They have a multitude of events that led up to them, and cause a myriad of events afterwards. To truly understand an event it is necessary to explore the hidden 90%. The same with mental health clients, presentation is also like that top 10%. It is also frequently necessary to explore the client's past and present to put behavior into perspective.
In psychology the iceberg model is commonly used to describe Freud's 3 levels of consciousness.