Support explaining how this code works

HI everyone, 

Could you help explain me how below code works? The character "_" is new to me and I cannot find any documentation for it.

 

apply(
        contains(
          index(
            local!childTask,
            "id",
            null
          ),
          _
        ),
        index(
          local!parent,
          "id",
          null
        )
      )

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