An order block is the last candle (or cluster of candles) that moved against the direction of a subsequent strong impulse — the final down candle before a sharp rally is a bullish order block; the final up candle before a sharp decline is a bearish one. Its high-to-low range is marked as a zone where price may react when it returns.
The narrative, from ICT and smart-money-concepts trading, is that large participants built positions in that candle, so unfilled interest remains there. Whatever the mechanism, the testable claim is simple: price revisiting the zone of the last opposite candle before displacement reacts there more often than chance.
This is among the most discretionary of ICT concepts. Variants disagree on whether the zone is the full candle range, body only, or open-to-extreme; whether the impulse must break structure or leave a fair value gap; and when a block is invalidated — so precise rules must be fixed before any honest backtest.