An order block is the zone of opposite candles that built up before price reversed and closed beyond the start of that run. Picture a small series of green candles pushing up, then a big red candle that closes below the first green candle's open. The moment that red candle closes, the bearish order block is the zone from the first green candle's open down to the last green candle's close. Flip the colours for a bullish block: the red candles before a green close that reclaims the first red's open.
For education only. Nothing here is financial advice, and trading involves risk.