On this example, we could notice a very good situation. Indeed, the price is stopping to go down, and start to be sideways (in range). Suddenly, you could see 4 big green point which are triggered, and appears on the quantmap with the text label "Bid Iceberg".
What does it mean ?
It means that there are no a lot of liquidity on the bid side of the orderbook, and especially on the best bid. But it is what you see in the orderbook with your human eyes. And indeed, when we have a look on the bottom of the chart, we could see the green band which are in dark green at this moment ; it means that there is no "large" bid stored here (on the best bid).
But, if there are no large liquidity on the best bid, why the iceberg orders is triggered?
It is triggered because some body, or some thing (algo HFT machine for example) is adding/removing a quantity of limit orders which is bigger than the last sold contract which has hit the bid. The best bid is replenished bigger than the market order quantity. It could be reached in time space till 1 milli seconds.
Also, when you have this kind of HFT alert (Bid iceberg for example) it is showing to some hidden activities through the orderbook. It is very usefull to see the real and true interest from the orderbook large players.
Last point that we have to notice, is that these iceberg orders are just triggered around the vpoc of the day... (volume point of control) ; it is the key price area, where there was the largest quantity of volume (by price) which was printed in.