Direct Answer
llms.txt is a plain-text file at the root of a website (like robots.txt) that gives large language models a curated, structured map of the site's most useful pages. It is not officially part of any standard yet but is increasingly respected by AI crawlers and helps engines decide what to summarize and cite.
Voice answer (≤30 words)
“llms dot txt is a file at the root of a website that gives AI models a curated map of the most useful pages.”
Pair llms.txt with llms-full.txt, which contains the full canonical content of the most important pages in plain text.
Yes, every site should have one. It costs an hour to write and signals to AI engines that you are an organized, citation-worthy source.