You'll be able to use the app, which will eventually be available on iOS and other Android devices, to manage your account and controllers, buy titles and start games on Chromecast Ultra. Once you've signed in with a Google account, it'll ask you for an invite code from the Stadia Founders or Premiere Edition package before you can go any further.
The Play Store listing notes that you can access your games library from the Home tab. An Explore tab, meanwhile, seems as though it'll feature community posts as well as links to Stadia's presence on YouTube, Reddit, Twitter, Facebook and Discord. You'll also be able to see which of your friends are online and what they're playing.
Stadia will work with some Chrome OS tablets, Pixel 2, Pixel 3, Pixel 3a and Pixel 4 at launch, along with browsers and Chromecast Ultra. The service will be available more broadly next year.