Roads are designed to carry a certain number of vehicles to keep them all moving at a reasonable pace. That's their "capacity." If more cars than that show up during rush hour, the road can't refuse them*. It's carrying more than its nominal capacity. We get traffic jams that may stretch for miles. As the rush hour winds down and traffic drops to a tolerable level, the jams ease up and the road works well again.

Same with airports. Their "capacity" is the number of passengers they can handle during normal flight hours without undue delays or crowding, however the airport designers defined that. If more flights, people, bags, etc., show up, it gets overcrowded and things slow down - perhaps a lot. Everybody will be out by 2 am, though.

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*A few highways control access via lights during peak periods. Those are an exception.