Before the age of radio, ships at sea would communicate using signal flags. Each flag was unique, and had a different meaning for everyday civilian use, either as a letter or as a code.
In navies, however, in order to keep movements and strategies a secret, the signal flags would take on their own coded meanings, with a codebook kept aboard ship, not too dissimilar from coaches and players giving signs in baseball. In today’s game, there were plenty of hoisted flags warning the Mariners of what was to come, plainly obvious to us watching the game.