It was published as a proposal in 2024: an /llms.txt file with a short description of the website and links to its important pages, meant to help a model find them without reading the whole navigation.
It is not a standard adopted by search engines, and Google says its AI features need no special file. It may help tools that read it. It does not replace a crawlable website, a sensible robots.txt or pages that answer.
If you publish one, make it say what your website says. An llms.txt that promises what the pages do not is just another kind of inconsistency.