https://www.cnet.com/news/google-discontinues-its-chromecast-audio-device/

The Chromecast Audio is a dongle that you can plug into to your speaker to add Wi-Fi connection and the ability to stream audio -- essentially turning your non-connected speaker into a "smart" speaker. It looks and works a lot like the standard Google Chromecast, except it connects via auxiliary cord instead of HDMI and only streams audio.

The device was first announced in 2015. Since then, Google has released various smart speakers including the Google Home, Google Home Mini, Google Home Max and Google Home Hub.