To monitor a Kubernetes cluster effectively, it's essential to set up a monitoring tool that can collect application metrics from target pods or services. Prometheus is an excellent open-source tool for this purpose, and setting it up on a Kubernetes cluster can be achieved using the kube-prometheus-stack Helm chart. After installing Prometheus, it's crucial to configure the app to expose metrics, which involves writing manifest files and applying them to the cluster. The Prometheus UI provides a visual representation of the metrics, while Grafana is used to visualize these metrics on interactive dashboards. By setting up alerts using Prometheus and Grafana, developers can detect and respond to issues proactively, ensuring the smooth operation of their applications and leading to better reliability, performance, and availability.