AMQPHelpers.Adapter.ack

You're seeing just the callback ack, go back to AMQPHelpers.Adapter module for more information.
Link to this callback

ack(channel, delivery_tag, options)

Specs

ack(
  channel :: AMQP.Channel.t(),
  delivery_tag :: integer(),
  options :: keyword()
) :: :ok | {:error, term()}

Acknowledges one or more messages.

See AMQP.Basic.ack/3.