What it checks
robots.txt is a plain text file at the root of your site that tells search engine crawlers which paths not to crawl. The tool reads the "User-agent: *" group, which applies to every crawler. If that group says "Disallow: /", the entire site is closed to crawling.
Why it matters
That line is usually put on a staging site on purpose and forgotten when the site goes live. Google stops crawling: pages may linger in the index, but their content is no longer refreshed and new pages are never added. Traffic falls quietly over weeks rather than overnight, which is why it is noticed late.
How to fix it
- Open the file in a browser: yoursite.com/robots.txt.
- Remove the "Disallow: /" line under "User-agent: *", or list only the paths you mean to close (for example "Disallow: /wp-admin/").
- Check Search Console's robots.txt report to see which version Google fetched, and request a recrawl.