Market depth is a metric, which is showing the real liquidity of the markets. Due to rampant wash-trading and fake activity - volume currently isn't the most reliable indicator in the crypto space.
What is it measuring?
It's measuring 1% or 10% section of the order book from the midpoint price (1%/10% of the buy orders, and 1%/10% of the sell orders).
Why it is important to use only 1% or 10%?
It's important, because measurement of the whole order book is going to give false results due to extreme values, which can make false illusion of liquidity for a given market.
How to use it?
By default Market depth is showing the most liquid markets sorted by Combined Orders (which is a sum of buy and sell orders). This way it provides the most interesting information already. Left (green) side of the market depth bar is showing how many buy orders are open, and right (red) side of the bar is showing how many sell orders are open (both can be recalculated to BTC, ETH or any fiat we have available on the site).