Icebergs hide size. A trader who wants to buy 100,000 shares might show 500 at a time so the order-book does not reveal the full demand and push price away.
You can sometimes spot them on time-and-sales: repeated fills at one price while the displayed size on level-2 never seems to shrink.
Example: the ask shows 500 shares at $30.10. Ten thousand shares print at $30.10 over two minutes and the offer still shows 500. Someone is refreshing an iceberg.
Related: order-book, level-2, time-and-sales, limit-order