A Wifi device to connect to your stereo to play music from your smartphone or tablet.
It works over WiFi so has a longer range than Bluetooth.
It also has better quality than bluetooth.

Output:
  3.5 mm sterio plug
  3.5-mm-stereo-to-RCA cable
  Mini TOSLINK-to-TOSLINK optical cable for digital output.

The Google Home app Android iOS Just click the Chromecast icon in the app to start playing.
It will keep playing even if you close the app and turn off you device.
You can select the icon and select disconnect to stop.
See Apps - Chromecast - Google

Configure the Chromecast Audio: Get the Google Home App Note: On Apple iOS make sure you specify "Google Home" not just Home which select "Apple Home"
It will discover new devices, Select your device then a network. Enter the network password.

Android:
Once configured as far as I can tell any mobile device that is on the same network can play thru it. (In my case I have an ac router with a 2.4 GHz net and a 5 GHz net and it works with either network.)
On android you can click the hamburger menu in the Android Google Home App on an android device to send all audio to Chromecast.
I couldn't get it to work with my Verizon LG Tablet.

iOS:

It works from ChromeCast Audio enabled apps.
e.g. Pandora, Spotify, Google Play Music, YouTube Music, iHeartRadio, ...
See Apps - Chromecast - Audio (www.google.com/chromecast/audio/apps/)

Select the chromecast icon in the app.
Select the ChromeCast device.
It is sometimes difficult to get to the right screen where the cast icon is displayed.

You cannot play audio from other apps like iTunes directly from your device.
You can use the AllCast app to do it.

Links:
Chromecast audio configuration at McBride Cabin
Set up Chromecast Audio - Chromecast Help
Chromecast Audio review - CNET
Chromecast tips, tricks and secret features