C-G: It plugs into your stereogram (rather than HDMI) so that you can stream music to it.
Saves running a lead from your laptop to the stereogram.
Only problem is, not all music playing apps are compatible, so check the one you use is before buying.
I’ve got one, but haven’t found it very useful really. It hangs there out the front of my stereo, just us ugly as if I just had a phono lead there – And the Chromecast has to be plugged into the mains all the time. And sometimes it doesn’t work when you want it to, which can be quite annoying.
(I’m sure others have found it fantastic and completely reliable – Which is why I posted that, for some balance, not to get into an argument about it being the best thing since sliced …etc.)