Use of Source in Shortcuts

[m90att]

Hi, I'm struggling to understand the use and the example of source in the shortcuts...

Definition:
"A source/(sound mode) name can also be specified to change source/(sound mode), use together with type source/sound_mode. "

Example:

https://github.com/kalkih/mini-media-player/blob/master/README.md#card-with-media-shortcuts:~:text=power_state%3A%20false-,Card%20with%20media%20shortcuts,-You%20can%20specify

      # Change the source to bathroom
      - icon: mdi:dog
        type: source
        id: Bathroom

Can someone tell me what this does and what the example is doing please? I don't understand what Bathroom is.

Thanks, Matt