It's a hand-written map you control, read first by AI agents crawling your site. For a store, it lists what you sell, your best pages and your policies in machine-friendly markdown. It complements — never replaces — robots.txt (access) and sitemap.xml (enumeration).
Note it's an emerging convention: Shopify serves one natively as of 2026, while Google has called it "speculative." Cheap to add, useful, but not a ranking guarantee.
At your domain root, reachable at https://yourdomain.com/llms.txt, served as plain text. Generate one free with the AgentReady llms.txt generator.