The ads.txt file is a key component of all programmatic advertising and at this point, basically all demand partners (SSPs) require that it be used and maintained properly. You can read the specification for it from the IAB Tech Lab.
This file needs to exist in the root folder of the apex domain, and each line in the file represents an advertising partner that is authorized to actually buy ads on that domain.
For example, if you are running Example Domain and want to run programmatic ads, the following URL needs to either load directly, or redirect to another URL that loads directly (it should not result in multiple redirects):https://example.com/ads.txt
There are two ways to manage your ads.txt entries with Organic Ads. We recommend using the redirect method, as it provides for the fastest management and the easiest integration, but we support the server synchronization method in case you have a complex need to automate the lines in ads.txt.