There are no periods in SOS. It means “help” but it does not stand for anything: not “save our souls”, or “save our ship”. It was first used by Germany, so who knows what it might stand for. It was selected because it is easily recognized in Morse code (dot dot dot dash dash dash dot dot dot) back when all radio communications were in Morse code. It replaced CQD (dash dot dash dot space dash dash dot dash space dash dot dot) (this damn app won’t let me use – and .) which was much more difficult to recognize if you weren’t paying attention.