The server is currently unavailable (because it is overloaded or down for maintenance).
What does HTTP 503 mean?
The 503 Service Unavailable code is part of the 5XX class. This is a server error code — the request was valid but the server failed to fulfil it. The problem is server-side.
When is it used?
A server returns 503 Service Unavailable in situations matching the definition above. You will encounter it inspecting network requests, reading logs, or testing an API. Whether it signals a problem depends on its class: 2xx and 3xx are normal, while 4xx and 5xx usually need attention on the client or server respectively.