A buy wall is a disproportionately large buy limit order placed on a cryptocurrency exchange.
The vast majority of cryptocurrency exchanges facilitate deals between traders through the use of an order book — a collection of buy and sell orders placed by traders. An individual order is a request to buy or sell a certain amount of cryptocurrency at a certain price.
The exchange then helps buy and sell orders with matching prices to mutually execute — partially or in full depending on the number of coins listed in each of the orders.
A buy wall is a situation where a large limit order has been placed to buy when a cryptocurrency reaches a certain value. This can sometimes be used by traders to create a certain impression in the market, preventing a cryptocurrency from falling below that value, as demand will likely outstrip supply when the order is executed. Often a buy wall is created by a buy limit order for a disproportionately large amount of coins placed by a cryptocurrency whale.
In a way, it “blocks” the price of the asset from going below a certain threshold: there is no rational reason for sellers to offload their assets at a price lower than what the already existing unfulfilled buy wall offers to purchase at. Buy walls can be created for different purposes.
One of them is simply to acquire a large amount of a certain cryptocurrency in cases where the buyer is bullish on it.
Another possible reason for placing a buy wall is price manipulation — a whale who holds a large amount of a certain cryptocurrency might be concerned with the public image of their asset. Consequently, they use their resources to simultaneously buy more coins and prop up the price, making the asset appear more healthy than it actually is.