In practice
A volume imbalance is a small gap between the close of one candle and the open of the next, where the wicks traded but the bodies did not. It is a finer version of the same idea as a fair value gap.
Traders use it to extend or refine other zones, for example treating an order block and the imbalance next to it as one area, and expect price to trade back through it.
What to watch for
These are tiny and very common, especially on low timeframes and in markets that gap between sessions. Alone they are not a reason to trade.