503 Service Temporarily Unavailable是什么?

503 Service Temporarily Unavailable是什么?
Service Temporarily Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
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Apache/2.0.46 (Unix) PHP/4.3.2 mod_fastcgi/mod_fastcgi-SNAP-0302032017 Server at www.sdau.edu.cn Port 80
怎么做成的?