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.

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).

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.

Historical Market Depth is showing the history of liquidity from the markets for a given asset. It’s a measure of combined liquidity from all integrated markets on the coinpaprika’s market depth module.