What does it all mean, Basil?
That means someone took 1.5 billion HBAR off of the exchange and moved them to their "Cold Storage" wallet.
If you are going to sell or trade you leave them on the exchange - you usually do that if you think price is going down and you need them on the exchange to sell them.
Opposite is true when you take them off the exchange - you think the investment/project is good and the price is going to go up - so you move them to the safety of your "cold" storage wallet.
Think of it this way...
If you own a piece of art and think the value might go down or you want to sell it you have to have it with "Sotheby's' or an auction house to sell it - these are like crypto exchanges they are the place you have to have the artwork to sell it in the open market.
If you think the value of your art is going up and don't want to sell it you keep it in your home or other secure place - think of your home as the "Cold Storage Wallet"
Basically someone just took $329,000,000 worth of HBAR home with them because they are going to hold it as they anticipate the value going up and don't want to sell now.
Someone with a shit-pile of money/HBAR just indicated they think it's time to sit at home and enjoy the ride.
Hopefully the above helps.