What does “kubectl set env daemonset aws-node` mean?

...don't understand what subcommand this is: daemonset aws-node -n kube-system CLUSTER_NAME=${YourClusterName}

Every EKS node runs an instance of aws-node pod which provide CNI functionality to the node. The command set the environment variable named CLUSTER_NAME value to the name of your cluster for every pod managed by the daemonset. So that you do not need to go node by node to change each running pod environment variable, the command set and automatically restart all the pod(s) for you.