I will advise you to first explain Flapping first. Flapping occurs when a service or host changes state too frequently, this causes lot of problem and recovery notifications.
Once you have defined Flapping, explain how Nagios detects Flapping. Whenever Nagios checks the status of a host or service, it will check to see if it has started or stopped flapping. Nagios follows the below given procedure to do that:
Storing the results of the last 21 checks of the host or service analyzing the historical check results and determine where state changes/transitions occur
Using the state transitions to determine a percent state change value (a measure of change) for the host or service
Comparing the percent state change value against low and high flapping thresholds
A host or service is determined to have started flapping when its percent state change first exceeds a high flapping threshold. A host or service is determined to have stopped flapping when its percent state goes below a low flapping threshold.