On a candle chart an imbalance appears where one bar's low sits above the previous bar's high, or vice versa, leaving a vertical band that was crossed without overlap. The same idea is called a fair-value-gap in market-structure vocabulary and a single print in market-profile.
The reasoning is that price moved through the area without both sides transacting, so unfinished business remains there. Markets frequently return to trade through such zones later, which makes them plausible targets and plausible reaction areas.
Frequency is the problem. Fast markets create dozens of these bands, most are filled quickly and mean nothing, and picking out the ones that mattered is usually done after the fact. Combine with size, volume, and location before treating one as significant.
Related: fair-value-gap, single-prints, displacement, gap, wide-range-bar