What is ads.txt?

IAB Tech Labs introduced ads.txt files to protect publishers against fraudulent sellers and unauthorized inventory sales. 

An ads.txt file is a simple text file, not an embed code, that is uploaded into your domain’s root inventory. In other words, it is implemented the same way a robots.txt file on your root domain would be implemented.

Additional information can be found at https://iabtechlab.com/ads-txt/.