You can answer this question by first mentioning that Nagios is one of the monitoring tools. It is used for Continuous monitoring of systems, applications, services, and business processes etc in a DevOps culture. In the event of a failure, Nagios can alert technical staff of the problem, allowing them to begin remediation processes before outages affect business processes, end-users, or customers. With Nagios, you don’t have to explain why an unseen infrastructure outage affect your organization’s bottom line.
Now once you have defined what is Nagios, you can mention the various things that you can achieve using Nagios.
By using Nagios you can:
Plan for infrastructure upgrades before outdated systems cause failures.
Respond to issues at the first sign of a problem.
Automatically fix problems when they are detected.
Coordinate technical team responses.
Ensure your organization’s SLAs are being met.
Ensure IT infrastructure outages have a minimal effect on your organization’s bottom line.
Monitor your entire infrastructure and business processes.
This completes the answer to this question. Further details like advantages etc. can be added as per the direction where the discussion is headed.