What an order block is
An order block is a concept from price-action analysis that points to one specific origin: the compact candle range that immediately preceded a strong, sustained move in a single direction. What defines it is the causal story attached to that exact range. The idea is that a concentration of buying or selling was committed there, and that this footprint is what set the subsequent move in motion. The concept is about identifying that originating range and the price action that came out of it, not about cataloging every level where price once paused.
How the concept is applied
A bullish order block is typically the last down-candle range before a strong rally began; a bearish order block is the last up-candle range before a sharp decline. Traders who use the concept mark that single range and watch whether price returns to it and how it behaves there. The emphasis is on the strength and immediacy of the move that followed the range, which is what practitioners use to distinguish an order block from an ordinary pause in price. Definitions vary widely between practitioners.
Limitations to keep in mind
Order blocks are subjective and are not standardized indicators. There is no way to confirm from a chart alone what orders actually sat at a range, so the term describes an interpretation of past price behavior rather than a verified fact. It is one lens among many and does not forecast future movement or guarantee any reaction if price returns.