What is the center line of the cargo ship?

The lines you are referring to are called the Plimsoll mark, named after the gentleman that came up with the idea. They are present because in the early days of sailing and insurance. Unscrupulous companies would overload their vessels with cargo. The premise being if they completed the trip, they would do well, if they did not they were insured for the loss.

So Plimsoll decided to establish a line of the side of the vessel so that insurance inspectors (and incidentally everyone else) would know simply by sight if the vessel was loaded safely.

This had endured to modern times. You will see many different lines on the side of a vessel, they represent different areas of the worlds oceans that the ship may sail. The Highest or light loads are to carried in the North Atlantic Winter. With tropical locales allowing the ship to be loaded slightly heavier.