Nagios is an open-source monitoring system for computer systems, networks, and infrastructure. It provides comprehensive monitoring and alerting capabilities, allowing administrators to detect and respond to issues before they affect users. Nagios can monitor various aspects of IT infrastructure, including server resources, network protocols, applications, and services. It supports custom plugins and integrates with numerous third-party tools, making it highly flexible and extensible. Nagios's alerting system can notify administrators via email, SMS, or other communication channels, ensuring timely awareness and resolution of issues.