An order block is the last opposing candle (or consolidation) before a strong directional move — the last down candle before a big rally (a demand zone), or the last up candle before a selloff (a supply zone). The theory is that institutions positioned there before pushing price, so the zone matters on a retest. Used honestly, it's a refined support/resistance concept.
The plain-English version
Forget the mystique about “where banks fill orders.” What's actually true: a zone that launched a strong move is a place where demand (or supply) overwhelmed the other side. When price returns to that zone, the same imbalance often reasserts — buyers who missed the first move step back in at the demand block. It's closely related to a high-volume node or the origin of a break of structure: the spot the move came from.
How to use it
Treat an order block as a support/resistance zone for a retest entry: after a strong move up, a pullback to the demand order block (the last down candle before the rally) is a potential long, with a stop below the block. It's a retest concept with a specific origin. Look for a reaction at the block, not a blind entry into it.
An order block is just the launchpad of a strong move — the zone where one side overwhelmed the other. Price often returns to it and the imbalance repeats.
The honest caveats
Order-block trading is subjective (which candle is “the” block?) and dressed in jargon that oversells it — it's structure, not a signal, and it fails plenty. It's most reliable when the block coincides with something objective: a dealer level, a high-volume node, VWAP. An order block sitting on the put wall is a real zone; one floating in mid-air is a guess. Use the concept, keep the skepticism, and anchor it to the map.