Orders with Icebergs are only partially visible in the order book. The amount shown is equal to the full amount divided by the number of Icebergs. For example, if you have an order with 10 Icebergs, a maximum of 10 percent of the order size will be visible at all times.
Use Icebergs if you need to get a big order filled and don't want to show your full intention. Use Hidden orders if you don't want to show any intention at all.