An accumulation of asks or sell orders on an order book at a particular price level, serving as resistance.
Sell walls, akin to buy walls, are made of up limit sell orders accumulated around a certain price level.
Just like a buy wall, a sell wall can also be removed until filled, and is often used by traders to identify potential resistance levels for the price.
If a sell wall is valid and remains on the order book, an asset’s price would have to absorb all the selling and pierce through that wall to rise higher. Failing to do so, would result in the price getting rejected from that level.