What does the 503 error mean?
The 503 error can appear as Error 503 Service Unavailable and is an HTTP response code indicating that the server is not ready to handle an HTTP request.

Mostly, this error is caused by server maintenance or server overload. During maintenance, server administrators may temporarily route all traffic to the 503 error page, or this may happen automatically during software updates. In cases of overload, some server applications will reject requests with the 503 error when resource limits such as RAM, CPU, or connection limits are reached. Rejecting requests with the 503 error prevents the server from being pushed to its maximum limits, thus avoiding failures and crashes.

How to resolve the 503 error?
As a website user encountering the 503 error, if there is no further explanation (such as the reason for this HTTP response), you should contact the website owner/administrator, as they might not be aware of the server status.

As a server administrator, when the server is under maintenance, it's recommended that traffic be routed to a custom 503 error page, which mentions the cause and the estimated duration of the maintenance. If this error is returned due to overload, server administrators should consider upgrading the server, by adding more RAM and replacing the processors with more powerful ones, which can theoretically handle the increased resource demands.