The -- option in bash commands, as in tox -- -s, says there are no more options, and the things that follow are arguments. It’s also really hard to Google for, and so I only learned what it meant while reading a book on shell scripting.

It’s also in man bash’s OPTIONS section

   --        A  --  signals the end of options and disables further option
             processing.  Any arguments after the -- are treated as  file-
             names and arguments.  An argument of - is equivalent to --.

So for example:

test_program --awesome  # awesome is an option for test_program
test_program -- --awesome  # "--awesome" is an argument

As an example use case, I might need to collect commands and pass them to a child script:

test_program -- --awesome
# test_program calls other_program with the options --awesome

If test_program also did something with --awesome, then the -- helps tell other_program --awesome, instead of test_program.