Since each network device (or "server") usually has several parameters or services that should be monitored you can set up various sensors to monitor each and every aspect of a server (e.g. CPU load, webserver responsiveness, SMTP responsiveness, PING time, etc.) So a "server" has several "sensors" by which you will monitor all aspects of the server.
Each sensor is shown with a color depicting its current state, e.g. DOWN, UP, PAUSED, or OK.

See Managing Sensors for more information on adding and editing sensors.