A three-digit number by which a server responds to a request. 2xx (200) = OK. 3xx (301, 302) = redirect. 4xx = an error on the request side: 403 access forbidden, 404 page not found, 429 too many requests. 5xx (500, 503) = a server-side error.
For more detail see Errors 403, 404 and 500.