Semaphore it's NOT! Them's International Code of Signals flags, each of which, apart from its individual letter identification, has a very specific meaning (eg "O" = Man Overboard). When combined in specific groups of two, three or four, there is a whole code of internationally agreed messages.
I used to know them off by heart in my 'old salt' days but these days I would need to look some of them up.
(Semaphore is the flag in each hand held at specific angles to the body)
Like this: