Flag semaphore is the telegraphy system conveying information at a distance by means of visual signals with hand-held flags. Information is encoded by the position of the flags; it is read when the flag is in a fixed position.
Semaphores were adopted and widely used in the maritime world in the 19th century. Semaphore flags are also sometimes used as means of communication in the mountains where oral or electronic communication is difficult to perform.
The Japanese merchant marine and armed services have adapted the flag semaphore system to the Japanese language.
Because their writing system involves a syllabary of about twice the number of characters in the Latin alphabet, most characters take two displays of the flags to complete; others need three and a few only one.
Via Wikipedia for more on the history and the full “alphabet” for both Latin letters and numbers, and the Japanese version as well.