AR Camera Background Explain?

I don’t understand what this description is saying. Can someone explain it to me?

  • What does "render the color camera texture to the background" mean
  • What does “subscribing to the AR Camera events and blit the Camera Texture to the screen” mean?

Thanks!

This works fine for ARFoundation 2.1 but in later versions the “Use custom renderer” button does not appear. With Unity 2019.4, it doesn’t work. That is, you don’t see the real world as a background. Earlier versions of ARFoundation work fine. Can anyone help please?