The Merchant API is a new development in Bitcoin, released in April 2020.
The API enables merchants accepting Bitcoin transactions to get all of the assurance they need to be able to accept transactions on a 'zero confirmation' basis (see Confirmation). Merchants are able to find out in advance (even before broadcasting a transaction) what transaction fee is required by miners to mine it.
As explained by Steve Shadders in Steve Shadders Discusses the Bitcoin SV (BSV) Tech Pillars - Bitstocks Podcast Ep. 7 at 36m 12s:
It also allows miners to put user based fee policies in place. See The dawn of the age of competitive mining (Steve Shadders, yours.org, October 2019)
The BRFC specification can be found here.
The reference implementation can be found here.