Demurrage kicks in when cargo is not collected or loaded within the agreed free days. Carriers and terminals charge per day on a rising scale: the longer it sits, the more the next day costs.
The usual causes are customs delays, document problems — a B/L not released — or no room in the buyer’s warehouse.
It is not the same as detention: demurrage runs while the full container is still inside the terminal and stops when it is collected; detention runs while the container is outside and stops when it is returned empty. Some carriers bill both under a combined tariff.
An unexpected demurrage bill can turn a positive margin negative, which is why a stalled container should be one of the first alerts a CTRM raises.