Iceberg orders are large transactions that are divided into a number of smaller limit orders, so as to conceal the transaction and avoid market repercussions from the execution of such a sizeable trade. These tactics are typically used by institutional investors attempting to take or unload significant positions without driving an asset’s price in an unfavorable direction.
Doing so is necessary because, when a large order is placed on an order book, it is publicly viewable and may take a period of time to be completely filled. In the time it takes for the order to be filled, the market will likely react to the increased supply/demand for an asset indicated by this order. If this were to happen, an investor looking to buy a large quantity of an asset would end up paying a price much higher when the order is eventually filled than was the market price when it was placed. Conversely, a trader attempting to sell a large quantity of a given asset will likely see a majority of their assets sold at a much lower price when their order is filled than when it was placed.
The term derives its name from the fact that only a small portion of a much larger iceberg is visible from above sea level.