A buy wall is a disproportionately large buy limit order placed on a cryptocurrency exchange.
An individual order is a request to buy or sell a certain amount of cryptocurrency at a certain price. For instance, say you want to buy Bitcoin. You will place a buy order on the exchange for the price you are willing to pay, and the quantity of Bitcoin you want to buy. The exchange will then place your order into the order book, which contains all orders made by other traders and is public for all to view. The exchange will then match your buy order with users who want to sell, filling the orders at each price point and quantity available. You can also choose to fill your buy or sell orders at the best possible market price, which the CLOB will immediately fill based on existing orders.
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.
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.