Ads.txt stands for Authorized Digital Sellers and it manages to ensure that your digital ad inventory is only sold through sellers who you've identified as authorized.
You can manually check if a website has an ads.txt file and view its contents by adding /ads.txt to the end of the root domain, e.g. website.com/ads.txt
That is, once a brand/publisher has added the ads.txt file to their website, it is possible to verify and validate that the brand/publisher is really who they say they are. If by chance the ID referenced in the file does not exist or does not match, this may indicate that it is an unauthorized brand (and the opposite is also true).
For these reasons, ads.txt is used, for example, in authenticity checks to validate the display of advertising banners (such as Google Ads).
In order for your website to be able to display the Google Ads banners, it is necessary that you enter the ads.txt correctly according to the ID provided by Google, so that it can validate its authenticity and allow them to be displayed.
Now you can add the 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 ads.txt:
https://support.google.com/adsense/answer/7532444?hl=en