Naval flags are a type of code used by naval vessels to signal words, codes, or even singular meanings. They are based upon the Latin alphabet and are comprised of 26 letters, 10 (0-9) number flags, and four substitute flags (complex to explain, they basically mean they repeat whichever letter the flag stands for. Fourth repeat = repeat fourth letter).
Individual flags have specific and standard meanings; for example, diving support vessels raise the "A" flag indicating their inability to move from their current location because they have a diver underwater and (more importantly) to warn other vessels to keep clear to avoid endangering the diver(s) with their propellers.
