... is S; --- is O. So, the message reads "SOS", the standard distress message.

Though some people believe this sequence of letters means "Save Our Ship" or even a romantic "Save Our Souls", the truth is that the famous abbreviation doesn't have any literary meaning. It is used because such a sequence of dots and dashes is simple enough and easily recognizable.

Have you known about this fact? Tell us in the comments!

More Info: en.wikipedia.org