Short answer
An llms.txt file is a proposed plain-text file that helps AI systems understand which pages, docs, or resources are useful to read. It is not a confirmed Google ranking factor, so it should not be treated like a shortcut for better rankings.
Where it can help
The file can be useful when your site has documentation, product pages, research, comparisons, or educational resources that you want AI crawlers and answer engines to understand more clearly. A good file points to important URLs, adds short context, and avoids making crawlers guess which pages matter.
What to include
Start with your most useful evergreen resources. Add canonical URLs, short descriptions, and clear sections. Keep it readable. Do not turn the file into a keyword dump.
SEO expectation
For classic search, keep prioritizing crawlable pages, internal links, structured data, helpful content, and technical quality. Think of llms.txt as an AI-discovery support file, not a replacement for SEO fundamentals.