From Traderpedia

An Iceberg order allows you to submit an order (generally a large volume order) while publicly disclosing only a portion of the submitted order. When large participants, such as institutional investors, need to buy and sell large amounts of securities for their portfolios, they can divide their large orders into smaller parts so that the public sees only a small portion of the order at a time - just as the 'tip of the iceberg' is the only visible portion of a huge mass of ice. By hiding its large size, the iceberg order reduces the price movements caused by substantial changes in a stock's supply and demand.