In short. Delta is the difference, for a candle or a price level, between the contracts bought at the ask and the contracts sold at the bid. A delta of +350 means 350 more contracts were bought aggressively than sold aggressively in that candle. It measures aggression, not direction: price can fall on positive delta when limit sellers absorb the buyers.
The reading comes from the pairing of delta and price: price up and delta up is a healthy move; price up on negative delta is buying being absorbed, price down on positive delta is selling being absorbed. Those divergences are the first thing an order-flow trader checks at a level.
Delta per candle is noisy on fast markets like NQ; traders smooth it with cumulative delta across the session, or read it only at pre-defined levels. The article on delta and cumulative delta on this site has the three divergence cases with charts.
Questions traders ask
What is Delta?
Delta is the difference, for a candle or a price level, between the contracts bought at the ask and the contracts sold at the bid. A delta of +350 means 350 more contracts were bought aggressively than sold aggressively in that candle. It measures aggression, not direction: price can fall on positive delta when limit sellers absorb the buyers.