Mac.sigil_m

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

sigil_m(arg, _)

View Source (macro)

Specs

sigil_m(Macro.t(), [byte()]) :: Macro.t()

allows you to use the convenient ~m sigil to declare a Mac address in its normal string form, instead of using the tuple form.

iex> import Mac
iex> ~m"06:66:F4:12:34:56"
{0x06, 0x66, 0xF4, 0x12, 0x34, 0x56}