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Yes, SOS is a morse code distress signal, in popular usage, SOS became associated with such phrases as "save our ship", "save our souls" and "send out succour". In fact, as you can see it stands for "save our savouries" (ok, it could be that Greggs are upset with the Governement's latest VAT increases!).

These may be regarded as mnemonics, but SOS is not an abbreviation, acronym or initialism. In fact SOS is only one of several ways that the combination could have been written, VTB, for example, would produce exactly the same sound, but SOS was chosen to describe this combination. SOS is the only 9-element signal in Morse code, making it more easily recognisable, as no other symbol uses more than 8 elements.

Consider yourself educated on Morse Code, folks. I am now away to put my pie and sausage roll in the oven!