What are HTTP status codes?
When a browser or app requests a web page or API, the server replies with a three-digit status code that says what happened. The first digit groups the code: 1xx informational, 2xx success, 3xx redirection, 4xx client errors, and 5xx server errors.
The ones you'll see most
200 OK— the request succeeded.301 Moved Permanently— the resource has a new permanent URL (important for SEO redirects).404 Not Found— the resource doesn't exist.403 Forbidden— you're not allowed to access it.500 Internal Server Error— something broke on the server.
Why they matter for SEO and security
Search engines use status codes to decide what to index — a wrong 200 on an error page or a 302 where a 301 belongs can hurt rankings. For security, consistent 401/403 handling avoids leaking which resources exist.