The accessory is a power brick that plugs into the Chromecast’s USB port, and—in addition to providing power—embedded in the brick is a full-sized Ethernet port. Just plug in and you can forget about ever having to push bits over Wi-Fi again. The adapter is $15 on the Google Store, and it supports up to 100Mbps.
Google has been continually updating the Chromecast with new tricks. Besides the usual swath of media content, you can now use it as a presentation device by pushing Google Slides documents, the Google Docs PowerPoint competitor, directly to a TV. The company is also planning to use it more as a gaming device by experimenting with dual-screen gaming content. All of that is going to take bandwidth, so it’s nice to have a hardwired connection as an option.
Update: Annnnd it’s sold out. The adapter seems to be US only for now, though UK visitors will see a UK version in the pictures. Apparently it’s possible to roll your own Chromecast Ethernet solution with a specialized USB OTG cable and very specific third-party ethernet adapters.