Fair value gap (imbalance)
In a three-candle sequence, the gap where the wicks of the first and third candles don't overlap; a zone a fast move left imbalanced, often filled back in.
An FVG marks an area price moved through one-sidedly, with little trading from the opposing side. The market usually fills such an "imbalance" partly or fully; that is why a return to an FVG in the trend direction is used as an entry zone.
Higher-timeframe FVGs are more reliable. Not every gap gets filled; an FVG that stays unfilled is a sign of strong momentum.