This refers to one of a series of lines i inscribed on the hull of a steamship and indicating the maximum depth to which that ship may be safely loaded, thus guarding against the possibility of accidents caused by overloading. The reason for more than one load line is that the danger level differs slightly according to whether it be summer or winter or sea water or fresh water.
All British merchant ships plus those of other countries using British ports also carry a general reference load line, known as the Plimsoll line. This was introduced and made obligatory by the Merchant Shipping Act 1874 as the result of the efforts of Samuel Plimsoll towards ensuring the safety of ships at sea. The line, represented by a circle with a line drawn through it, was the precursor of the grid of load lines appearing on the ships of virtually all merchant lines today. The load lines are determined by Lloyd’s in respect of all ships registered in the U.K. and any vessel loaded to a degree that defies these lines can incur heavy penalties in any part of the world where the regulations are accepted and enforced.
Reference: The Penguin Business Dictionary , 3rd edt.