HTTP 5xx status codes (500-599) indicate server-side errors - the server failed to fulfill a valid request. Common examples include 500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable. The 100 series are informational responses, 200 series indicates success, and 400 series indicates client errors (bad requests from the client side). When you see a 500 error, the problem is with the server or application, not with something the client did wrong.