Server Status Codes : There are 4 common status codes 200,404,301,302. Whenever a webserver serves any webpage, it writes or logs a status code in its own log file, for example., 1) Status code "200" - which means the page or resource was found. 2) Status code "404" - which means the requested resource was not found on the server. 3) Status code "301" - When a page is redirected permanently. 4) Status code "302" - When a page is redirected temporarily.
Well, requests are not only put in a log file but sent as part of the header to the requester (web browser, bot, etc). Making sure the correct code is returned is important for SEO. For example, you don't want to return 200 when it should be 404, or redirecting using a 301 when a page changes location. Really, you don't want any 404s, though; that is, pages not found.
There are two more codes 1) Status code "410" - which means the requested resource is no longer available at the server and no forwarding address is known. 2) Status code "503" - which means the server is currently unable to handle the request due to a temporary overloading or maintenance of the server.