Depth refers to the ability of a market for a specific asset to sustain large orders of that asset without the asset’s price moving significantly. The more open limit orders there are on both sides of an orderbook for an asset, the more depth that book has.
A depth chart is a graphical representation of buy and sell orders for a specific asset at various prices. A depth chart illustrates both sides of supply and demand to show how much of an asset you can sell at a particular price point.
A depth chart is split in the middle, which is the price of the asset during the last trade. It is also organized across the bottom by price.
On the left (green) side you have the lowest buy order (price) that buyers hope the asset will become so they can buy it affordably.
On the right (red) side you have the highest sell order (price) that sellers hope the asset will become so they can sell it for a large profit.
Overlapping Values are Arbitrage
Notice the overlap in the green and red lines. In this example, there are buyers willing to buy at a price up to $5,996/BTC and sellers willing to sell at a price down to $5,983/BTC.
The overlap between $5,996 and $5,983 is possible because of SFOX's aggregated orderbook from many global exchanges and liquidity providers and is an arbitrage opportunity to SFOX traders.
Buy walls are a large number of buy orders, typically placed on the order book all at once, below the current price.
Notice the large number of green bids/buys on the left side of the image at $671/BCH.
Sell walls are a large number of sell orders, typically placed on the order book all at once, above the current price.
Notice the large number of red asks/sells on the right side of the image at $6,000/BTC.