A file at a site's root that gives AI systems a curated, plain-text map of its most useful content — robots.txt's counterpart for comprehension rather than permission.
Where robots.txt says what a crawler may fetch, llms.txt says what is worth reading: a short description of the site and a curated set of links, in markdown, without the navigation and markup an agent has to strip out of ordinary HTML.
It is a convention rather than a standard, and adoption by major AI systems is still partial. The cost is one file, and the discipline of writing it — deciding what your site is actually for, in a paragraph — tends to be worth more than the file.