llms.txt was proposed in 2024 by Jeremy Howard of Answer.AI. It typically has a short summary of the site followed by links to key pages, with an optional llms-full.txt containing more detail.
It's a convention, not a standard that every AI crawler follows, but it's cheap to add.
Why it matters when you launch
An llms.txt that describes your product clearly makes it easier for AI assistants to summarise what you do accurately.