Kubelet on the worker node register the node on kubernetes cluster. When it recieves the request to load container/pod on a node, it requests the runtime engine to pull the required image and run the instance. Kubelet than monitors the status of the pod and conatainers in it and reports to the kubeapi service on timely basis.
Kublet has to be manually installed on all the worker nodes. Kubeadm does not install it by default.