Semaphore is a method of visual signaling. It is most commonly done by the use of flags but can also be done with lights. Before the invention of the telegraph, signaling from high towers was used to transmit messages between distant points. Nowadays, it is used in sea as a means of communication between ships. Each flag indicates a different alphabet letter.
The flags used are the red and yellow Oscar flags.
And don’t forget:
Special reference can be made for BRAVO ZULU, which although sounds funny and puzzling by people not familiar with sailing, it means “Well Done” and is represented by the two-flag signal: