5xx status codes are HTTP response codes that indicate a server side error, meaning the server failed to fulfill a valid request. A common example is 500 Internal Server Error, which is a general signal that something went wrong on the server. Another is 503 Service Unavailable, which often appears when a site is temporarily down for maintenance or overloaded and cannot handle requests.
These errors matter for SEO because they can block search engines from accessing your pages and frustrate users who land on them. If important URLs return 5xx codes, crawlers may reduce how often they visit your site and previously indexed pages can lose visibility over time. To keep things accurate and consistent, monitor server uptime, review logs for recurring errors, and make sure maintenance windows return the right signals. A short lived 503 during planned maintenance is usually better than a vague 500, because it clearly indicates the outage is temporary.