An order block is the range of the last candle in the opposite direction before an impulse that broke structure. The idea is simple: before a strong move up there was a final down candle, and its range is marked as the place the move started from.
The block itself is just a rectangle from that candle's high to its low. Everything interesting starts afterwards: what happens to the zone next.
Four states
Every block carries a state, and it changes as price moves around it.
Fresh — price has never entered the zone.
Touched — price has entered the zone without passing through it. It is worth counting the entries too: a block price has returned to several times is no longer the same situation as an untouched one.
Mitigated — price reached the far edge of the block, meaning it passed through the whole thing.
Broken — a candle closed beyond the block. From that moment the zone stops existing as a level of interest, and it may later turn into a breaker block.
Why "before the impulse" matters
Without the tie to a break of structure, an order block degenerates into "any candle before a move", and one of those exists before every bar. The tie supplies the meaning: we care not about any move, but about the one that changed market structure — that broke a previous significant level on a close.
That is why blocks do not appear everywhere on the chart, only where a BOS or CHoCH happened first.
How it differs from an FVG
Different things by nature. An FVG is emptiness, a stretch price skipped without trading. An order block is the opposite: a specific candle where trading did happen, and fairly actively.
They often sit side by side: the impulse that left a gap started from some candle, and that candle is the block. But they do not always coincide, and they are worth keeping apart. There is a separate comparison for the details.
What a block does not promise
Marking the zone says the move began here. It does not say price will come back, still less that it will bounce. It is a coordinate on a map of the past.