Ads.txt is an initiative developed by the IAB Tech Lab, aimed at combating certain types of ad fraud, mainly domain spoofing and illegitimate inventory arbitrage, and providing transparency in what is a highly opaque industry.
The file is served in the publisher's root level of their domain. It must be posted there by the publisher's webmaster and is essentially a publicly accessible record. The file specifically contains the following information for each demand provider (seller) the publisher uses in their ad configuration:
<SSP/Exchange domain>, <Your publisher ID for that provider>, <The seller's relationship with the publisher (DIRECT or RESELLER>, <The tag ID which is optional>