Kernel.sigil_S

You're seeing just the macro sigil_S, go back to Kernel module for more information.
Link to this macro

sigil_S(term, modifiers)

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Handles the sigil ~S for strings.

It returns a string without interpolations and without escape characters, except for the escaping of the closing sigil character itself.

Examples

iex> ~S(foo)
"foo"
iex> ~S(f#{o}o)
"f\#{o}o"
iex> ~S(\o/)
"\\o/"

However, if you want to re-use the sigil character itself on the string, you need to escape it:

iex> ~S((\))
"()"