What does Sell Wall / Buy Wall mean?
When looking at the price book of an exchange it is also possible to see a graphical representation of the current order book.
When there are a large number of buy or sell orders at a certain price point it resembles a wall and is thus known as a buy wall or a sell wall.
There is also the inference that the wall makes it difficult for the market to get beyond that point, just as a physical wall can block forward progress. See also Trading Walls.