Buy Walls occur on exchanges when a massive buy order or combination of several large buy orders that all have the same price in the order book for a specific cryptocurrency. The existence of buy walls has the effect of driving prices up even before it's filled because a cryptocurrency's supply will go down considerably once price hits the buy wall and orders are filled. Traders watch for buy walls on the order book as potential support levels, though they can also be spoofed by whales to manipulate sentiment.
Example
“A massive buy wall appeared at $28,000 on the BTC order book, giving traders confidence that the price had strong support at that level.”