When traders want to enter the market, they look at buy and sell walls to figure out the market mood. If there are more buyers than sellers, this will create a buy wall, indicating positive market sentiment.
Just like stock markets, crypto exchanges have order books. They register moves when both sellers and buyers enter the market. Buyers enter the market by placing bids - the highest price traders are willing to pay for an asset.
In turn, sellers enter the market by placing asks - the lowest price traders are willing to sell an asset for. Between these two opposite moves - buying (bids) and selling (asks) - order books create buy and sell walls. Both are visually represented as the volume of orders on each side. Commonly, order books are also called Depth Of Market (DOM).
DOM or order book visualized - the red side represents asks, while the green side represents bids, source: TradingView.com
From this follows that the green volume order you see on the left is the buy wall. The question then is, what is a buy wall in terms of its usefulness?
First, a buy wall tells us that bid orders have accumulated at a certain price level. If the trades are executed at that price point, the volume will drive up the price of the asset. Moreover, when the buy wall is higher than the sell wall, the supply of the asset is reduced.
Consequently, this creates scarcity, driving up the asset price even before the bids are fulfilled. Then, when traders are bolstered by market confidence due to a high buy wall, they place their bids even higher than the buy wall. As a result, this further drives up the asset's price.
Veteran traders know this well. If the particular crypto or stock has a small enough market cap, these whales can artificially create buy walls to manipulate the market. Therefore, a buy wall could represent an illusion. One where the asset price is sufficiently inflated for the whales to sell it at a higher point than when they bought it. Needless to say, this reflects the fundamental way a profitable trading is done - buying low and selling high.