HTTP
503 Service Unavailable
What it means
The server is up enough to answer, but it is refusing to handle the request right now. Common reasons are an overload, a planned maintenance page, or a service that has been taken offline on purpose.
Where you usually see it
Busy shops, sites during a release, and hosts that return 503 when a process limit has been hit.
What to try first
Wait and try again later. If the page mentions maintenance, take that at face value and come back after the window they named. If you run the site, check process limits, disk space, and whether you left a maintenance flag on.
When to stop
Reloading in a tight loop can make an overloaded service worse. A 503 is not an invitation to switch DNS to a random other host.