Difficulty Understanding Program

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I am going through Streams in Dave Thomas’s book and here I am having difficulties in understanding the following program.

For, example what is receive in sleep function and why to use it?
What is after in sleep function?

How this whole program is executed?

Why Dave Thomas suddenly jumped to such a complex program just after explaining the basics of Stream.

defmodule Countdown do
  # sleep mode
  def sleep(seconds) do
    receive do
    after
      seconds * 1000 -> nil
    end
  end

  def say(text) do
    spawn(fn -> :os.cmd('say #{text}') end)
  end

  def timer do
    Stream.resource(
      # start of next minute or number of seconds in a minute
      fn ->
        {_h, _m, s} = :erlang.time()
        60 - s - 1
      end,
      # wait for the next second
      fn
        0 ->
          {:halt, 0}

        count ->
          sleep(1)
          {[inspect(count)], count - 1}
      end,

      # nothing to deallocate
      fn _ -> nil end
    )
  end
end