It's a documentation-only (currently, I remember seeing posts on python-dev to add it as a python-level construct but I don't believe that's been implemented) to document positional-only parameters, a feature currently only available to "native" functions.
Basically, at the C level you can define positional-only parameters such that sorted can not be called as `sorted(iterable=xxx)`, and this was specified using a "/" in argument clinic (https://www.python.org/dev/peps/pep-0436/#special-syntax-for...) and now appears in some docstrings. This is pretty common in builtins e.g. you can't give a name to the first argument of `max`, it doesn't have one (except informally).
Anything before the / is positional-only (the name is informational but not definitional), between / and * it's both positional and keyword, and after * (or *args) it's keyword-only.
Basically, at the C level you can define positional-only parameters such that sorted can not be called as `sorted(iterable=xxx)`, and this was specified using a "/" in argument clinic (https://www.python.org/dev/peps/pep-0436/#special-syntax-for...) and now appears in some docstrings. This is pretty common in builtins e.g. you can't give a name to the first argument of `max`, it doesn't have one (except informally).
Anything before the / is positional-only (the name is informational but not definitional), between / and * it's both positional and keyword, and after * (or *args) it's keyword-only.