Mongo.drop_index

You're seeing just the function drop_index, go back to Mongo module for more information.
Link to this function

drop_index(topology_pid, coll, name, opts \\ [])

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Specs

drop_index(GenServer.server(), String.t(), String.t(), Keyword.t()) ::
  :ok | {:error, Mongo.Error.t()}

Convenient function that drops the index name in the collection coll.