HTTP codes
Service unavailable
The server is running but cannot answer right now, usually during maintenance.
Also called: service unavailable, maintenance, downtime
503 is the status code for a server that is up but cannot answer right now. Usually maintenance, or more traffic than it can take.
Where you meet it
It is meant to be brief, and unlike a 500 it tells search engines to come back later instead of dropping the page from their index.