The nagios user interface looks something similar to:
As can be seen, Nagios marks with green any service within acceptable “tolerances”. It marks in orange any warnings, and in red any critical services. In order to control when Nagios marks a service as “warning” and when as “critical”, most nagios plugins allow the use of the -w and -s swtiches which mean warning and critical respectively.
This is especially useful in SNMP, where a warning should be given if the SNMP value returned is above a certain threshold.