HTTP Status Codes (2xx/3xx/4xx/5xx)
HTTP status codes are three-digit codes with which a server answers a browser request, and their first digit sets the category: 2xx stands for success, 3xx for redirects, 4xx for client errors and 5xx for server errors. They are standardised in RFC 9110 and are essential for crawling, indexing and technical SEO.
In practice
For SEO practice a handful of codes matter most: 200 (OK) and 301 (permanent redirect) are unproblematic, while 404 (not found) is normal for individual old URLs but points to technical problems when it occurs in large numbers. It becomes critical with 5xx errors such as 500 or 503, because search engine crawlers may throttle crawling or temporarily drop content from the index after repeated server errors. 302 redirects (temporary) should also not be used permanently in place of a 301: Googlebot does follow them, but according to Google's documentation it does not treat them as a signal that the redirect target should be the canonical URL – the old address then stays in the search results.