Value	Status	Description
0	OK	The check went fine and everything is considered working.
1	Warning	The check is above the given warning threshold, or anything else is suspicious requiring attention before it breaks.
2	Critical	The check exceeded the critical threshold, or something really is broken and will harm the production environment.
3	Unknown	Invalid parameters, low level resource errors (IO device busy, no fork resources, TCP sockets, etc.) preventing the actual check. Higher level errors such as DNS resolving, TCP connection timeouts should be treated as Critical instead. Whenever the plugin reaches its timeout (best practice) it should also terminate with Unknown.

Keep in mind that these are service states. Icinga automatically maps the host state from the returned plugin states.