Why it’s important
A HTTP status code is a response from your server when a page is requested, whether that comes from a user or a search engine crawler. Status codes indicate whether the website page is served successfully, whether there are any redirects in place to other pages, or whether there are issues such as pages which are unable to be found or which are unavailable due to server errors.
A high number of issues and errors is obviously bad for website users, and will also lead to problems with search engines crawling and indexing your website effectively. There are a wide range of codes which can be potentially seen, and the most common ones are listed below.
Getting it done
2xx equals a successful process, such as 200, which indicates the server has correctly provided the requested page.
3xx is for redirections, such as 301, which indicates a permanent redirection to a new location, or 302 for a temporary move to a different location.
4xx indicates an error in the request, most commonly a 404 Not Found response, which visitors usually experience as a 404 error page.
5xx indicates a server error, such as 503 if the server is unavailable.
Response codes are listed in Google Search Console (formerly Webmaster Tools). under Crawl > Crawl Errors or via external tools such as Screaming Frog SEO Spider, which will crawl every page on your site.