Heapster is a cluster-wide aggregator of data provided by Kubelet running on each node.
This container management tool is supported natively on Kubernetes cluster and runs as a pod, just like any other pod in the cluster.
So, it basically discovers all nodes in the cluster and queries usage information from the Kubernetes nodes in the cluster, via on-machine Kubernetes agent.
https://kubernetes.io/blog/2015/05/resource-usage-monitoring-kubernetes/