The file that controls what search engines can crawl
robots.txt lives at the root of your domain (/robots.txt) and tells crawlers which paths they're allowed or not allowed to visit.
It uses a simple format: User-agent specifies which crawler, and Disallow / Allow specify paths. A Sitemap: directive tells crawlers where your sitemap lives.
Important: robots.txt is a suggestion, not a security mechanism. Malicious bots can ignore it. Never use it to hide sensitive content — use authentication instead.