Here - again - we
do have a sound that is the same and has two different
ways of being written. It's actually nothing special
just a simple s like in song or sauce. But as everything
German language has a history that makes this actually
redundant, but since we like tradition we keep it.
Then there is the orthographic rule when to use which
one of those. Actually you don't have to worry about
it for quite a long time. Since everyone will understand,
if you say that this letter is not on your keyboard
so you always can substitute it with two ss. Just that
you know, that there is a rule (after long vowels and
diphthongs is ß, after
short vowels the ss).