What is order flow in futures?

Order flow is the record of trades that already happened, plus the orders still waiting. Volume on a candle is the number of contracts that traded. For each contract there was one buyer and one seller. That is the whole market.

The order book shows contracts ready to trade at a price. Above the current price is the ask: passive sellers waiting. Below the current price is the bid: passive buyers waiting. The gap between the best bid and the best ask is the spread. A thick stack of contracts is a hard area for price to walk through. The common name for that picture is the depth of market, or DOM.

Most retail traders use market orders. A market buy lifts the ask. A market sell hits the bid. Aggressive hits passive. That is how a print happens. Liquidity, in this picture, is the resting size that can fill you. It is not a mystical pool.

Sources: How to actually trade order flow