Nautical Signal flags are used to communicate messages, at times because radio silence is required. Here are their names and meanings, both International and/or specific to the Navy, as well as meanings when used in a Sailing Regatta. There is also a signaling system called Semaphore which uses the position of the flags relative to a person to denote the letter.
This app will show you all the flags a-z and their meanings plus the number flags - everything is included.
Look at the flag - can you remember its meaning? press a button to reveal !