An HTTP status code is a three-digit number in the server response describing the result of the request. The first digit defines the class: 2xx — success, 3xx — redirection, 4xx — client error, 5xx — server error.
Most important codes
200 OK — successful request
201 Created — resource created (after POST)
204 No Content — success, no response body (DELETE)
301 Moved Permanently — permanent redirect
304 Not Modified — cached version is current
400 Bad Request — malformed data from client
401 Unauthorized — not authenticated
403 Forbidden — authenticated, but no permission
404 Not Found — resource does not exist
422 Unprocessable Entity — validation failed
429 Too Many Requests — rate limit exceeded
500 Internal Server Error — server-side error
503 Service Unavailable — server temporarily unavailable