Check out this video tutorial on how to add the ads.txt in 3 minutes.
The ads.txt is the code that you will have to add to the root of all your domains, in the ads.txt file, to approve our advertising to show and monetize on your domains. Hence, we kindly ask to update your domains' ads.txt files with the lines in this link.
Note that whenever we have an update on the ads code, we will let you know via email and we ask you to update your ads.txt accordingly. More info on the ads.txt:
Authorized Digital Sellers, or Google support.