An HTTP status code is a three-digit number that a web server includes in every response to summarize what happened with the request. These codes are grouped into five classes by their first digit: 1xx informational, 2xx success, 3xx redirection, 4xx client errors, and 5xx server errors. Reading them is the fastest way to diagnose why a page or API call behaved a certain way.
The most familiar codes are 200 (OK), meaning the request succeeded; 301 and 302, the permanent and temporary redirects; 404 (Not Found), meaning the resource does not exist; and 500 (Internal Server Error), meaning something broke on the server. Each tells a different story about whether the problem lies with the request, the URL, or the server itself.
For SEO and site health, status codes are critical signals. Search engines treat a 404 differently from a soft 404 that returns 200 with an empty page, and they handle 301s and 302s very differently for passing ranking. A 503 (Service Unavailable) during maintenance tells crawlers to come back later, while a mistaken 500 can cause pages to drop from the index.
Monitoring status codes at scale helps catch broken links, misconfigured redirects, and server outages before they hurt users or rankings. Checking a single URL's status, auditing a whole list of URLs in bulk, and following redirect chains to see the final code returned are routine tasks for anyone maintaining a large site or verifying a link list.