Well, exactly what it says.

The sever might be:
1) Rebooting; a standard maintenance reboot, so that the game keeps running smoothly (happens once a week).
2) Game Update; the sever is restarting so a game's update can be applied (you'll likely need to restart your game client, as well).
3) Technical Difficulties; the sever is down for some technical reason- maybe someone tripped on a power cord or something.

Either way, just wait for about 5-10 minutes or so, and the sever should be back up and running.