What does the signal S.O.S. mean?

SOS means help or mayday.

There is a rumor that it stands for "save our ship." This is not true. It was chosen because the Morse Code pattern - three dots, three dashes, three dots - is simple to do and easy to recognize. S.O.S. has been said to mean "Save Our Seamen", "Save our Ship", "Survivors On Shore" or "Save Our Souls", but these are just ways to help people remember the correct letters for the prosign. These phrases are actually backronyms -- acronyms created afterWord to help with memorization.