Nagios is an open-source monitoring system that helps organizations keep track of their IT infrastructure. It allows users to monitor servers, network devices, and applications to ensure they are functioning properly. If any issues arise, Nagios can send alerts to administrators, enabling them to address problems quickly and maintain system reliability. The software is highly customizable, allowing users to define their own monitoring parameters and thresholds. Nagios supports various plugins, which extend its functionality to monitor different services and applications. This flexibility makes it a popular choice for businesses looking to enhance their operational efficiency and minimize downtime.