Where it comes from
The format was proposed on llmstxt.org in 2024, for a simple reason: web pages wrap their information in navigation, ads and scripts, and turning that back into clean text is hard for a model with limited room to read. The proposal, now in its second version, says thousands of sites publish the file and that Chrome's Lighthouse audits sites for one.
What goes in the file
Only the H1 with the site's name is required. After it comes a blockquote with a short summary, then sections under H2 headings, each a list of links with a note on what each page covers. A section named Optional holds links an agent can skip when it needs a shorter read.
Does it help you get cited?
It depends on who reads it. Coding agents and assistants that fetch documentation use it. Google's documentation for AI Overviews and AI Mode says you don't need new machine-readable files or AI text files to appear in them. So treat llms.txt as a cheap, useful summary for agents, not as a route into Google's AI answers.