NGINX is a popular HTTP server and reverse proxy server that serves static content efficiently and reliably. It can also be used as a mail proxy, generic TCP proxy, or for caching and load balancing. Monitoring NGINX provides insights into resource issues within the server itself and problems in the web infrastructure. Key metrics to monitor include requests per second, server error rate, and request processing time. These metrics help identify spikes in incoming web traffic, errors, slow response times, and potential resource saturation. NGINX Plus offers additional metrics such as active connections by upstream server, 5xx codes by upstream server, and available servers per upstream group, which are relevant to a reverse proxy setup. By monitoring these metrics, users can gain visibility into the health and activity levels of their web infrastructure and identify potential issues before they become major problems.