https://www.cnet.com/news/red-dead-redemption-2-app-lets-you-explore-games-world-on-android-ios/

You can use the app to read the journal of the game's new hero, Arthur Morgan. Having the app also means you can remove the heads-up display from your TV when you're playing the game and still have Arthur's stats on hand.

The game and the app will be available for download Friday.