International maritime signal flags refers to various flags used to communicate with ships. The principal system of flags and associated codes is the International Code of Signals. Various navies have flag systems with additional flags and codes, and other flags are used in special uses, or have historical significance. Wikipedia

In harbour: All persons should report on board as the vessel is about to proceed to sea.

At sea: It may be used by fishing vessels to mean: "My nets have come fast upon an obstruction"