The daily charge that accrues once your free storage period at the port runs out - shipping line demurrage on the container, and port rent on the ground it is standing on.
It is the reason clearing is a race. A delayed permit, a query on the valuation, an agent waiting for your money: each of those is charged by the day, and the daily rate is not small.
Have the funds and the paperwork ready before the ship arrives, not after you get the notice.