Jump the line

To “jump the line” means to move ahead of people already waiting for something. Even then, it was not guaranteed Elizabeth would become queen. Her mother was still of childbearing age. If her mother had given birth to a son, then the son would have “jumped the line,” or the son would have moved in front of Elizabeth in the order of succession. The man was running late for his flight, so he “jumped the line” for airport security.