No, a Plimsoll line is not the row of French guns that the Germans bypassed in World War II, nor is it the weighted line that carpenters use.
Instead, it is a reference line that indicates the safe depth for a vessel loaded with cargo. A commercial ship is properly loaded when the ship's waterline equals the ship's Plimsoll line tells that it varies with temperature and the type of water.
Wikipedia tells that Samuel Plimsoll was a British politician, 1824-1898, and after suffering through poverty as a young adult, he devoted his life to helping the poor. One of those causes was the 'coffin' ships in which the owners recklessly exposed the lives of the crew by overloading them. Although his bill in Parliament was originally rejected in 1875 due to the many members of Parliment having ownership in shipping companies, it was successful a year later.
Today the Plimsoll mark is found midship on the port and starboard hulls of cargo vessels.