Different states are defined in nagios.sourceforge.net/docs/3_0/statetypes.html
The most simple configuration works something like this:
The service is normally checked in 5 minutes period (status OK )
Then something happens on that service!
Next check: Warning 1/3 soft state (nothing else happens)
Next check after 1 minute! Warning 2/3 Soft (nothing happens)
Next check after 1 minute! Warning 3/3 Hard state (notifications will be sent and possible scripts are started)