The spelling <ss> in the middle of a word represents the sound s most of the time, except when followed by <ion> or <ure> as in:
fissure impression mission passion pressure session succession
Here the pronunciation is ʃ. The same pronunciation is used for the words Prussia and Russia. The words scission and rescission may have either ʃ or ʒ.
The words issue and tissue can be pronounced with ʃ or sj.
There are a few words where surprisingly <ss> represents z
Aussie dessert dissolve hussar possess possession scissors
At the end of a word <ss> always stands for s.
With thanks to Petr Rösel for comments