Semaphore is a way to signal the alphabet using two flags, one held in each hand.
To send a message, your left and right hands have to be in two different positions. You start with both hands pointing down.
Here are the signals for the letters of the alphabet:
Image by kind permission of http://inter.scoutnet.org/
What does this message say?
You might want to send a message that contains more than just letters (exclamation marks, question marks, full stops etc). How many other symbols could you send using this code?