What this actually checks
This queries Google's own Safe Browsing database - the same list Chrome, Firefox, and Safari use to show their "dangerous site" warnings - for known malware, phishing, and unwanted-software listings against the URL you enter.
A clean result means Google hasn't flagged the URL as of this check - it is not a guarantee the site is safe, since new threats appear constantly and Google's crawl doesn't cover every page on the web immediately. Treat a "not flagged" result as reassuring, not definitive.
Why this uses the same database as your browser's own warnings
Google Safe Browsing isn't a third-party or proprietary blocklist - it's the actual database that powers the "Deceptive site ahead" and "Dangerous site" warnings built directly into Chrome, Firefox, and Safari. A site flagged here is one that would also trigger a full-page browser warning for most visitors, making this a direct, authoritative check rather than an approximation.
Why different threat types matter for diagnosing an issue
Malware, phishing/social engineering, unwanted software, and potentially harmful applications are each flagged for different underlying reasons - malware usually points to injected malicious code, while phishing points to content designed to deceive visitors into giving up credentials. Knowing which specific threat type was flagged narrows down what to actually investigate and fix.
What to do after fixing a flagged site
Being flagged doesn't resolve itself automatically once the underlying problem is fixed - site owners need to actively request a review through Google Search Console after cleaning up the issue, since Google's crawl doesn't necessarily revisit and re-clear a flagged site on its own schedule.