interval_length=60
This is the number of seconds per “unit interval” used for timing in the scheduling queue, re-notifications, etc. “Units intervals” are used in the host configuration file to determine how often to run a service check, how often of re-notify a contact, etc.
As I understand it -> if Nagios fails to check host or service it will reshedule it for the next interval so if you have max_check_attempts=5 configured this host will be checked for 5 times once a minute
Details are in the documentation under Timing Interval Length in the main configration file options