App-ads.txt or Authorized Sellers for Apps manages to ensure that your digital ad inventory is only sold through sellers who you've identified as authorized.
This means that it works exactly like Authorized Digital Sellers (ads.txt) however for mobile applications.
You can access the article referring to Authorized Digital Sellers - Ads.txt through this link:
Once the app-ads.txt file has been correctly added to the server, it is possible to verify and validate if a certain application has the compatible ID that works as the authorization that allows display ads in the app.
Now you can add the app-ads.txt content directly on the SEO Center tab of your Site Manager on Webmaster tools:
The link below (from Google.com) brings more information about app-ads.txt: