How does a plimsoll line work?

According to the International Load Line Convention, the Plimsoll mark is placed at the same level as the summer (or standard) minimum freeboard line. In standard conditions (summer, in sea water), if the vessel is loaded in such a way that the plimsoll mark is submerged, then it cannot sail. It is considered overloaded.

This is done, of course, to protect the lives of the sailors on board.