Maintenance mode explained
Some installation and updates that use live install require the host in the cluster to be placed in Maintenance mode. All virtual machines running on the host that is entering the Maintenance mode must be shut down or migrated to other hosts. After the host has entered the Maintenance mode, no VMs can be deployed or powered-on on the host.
To place a host in maintenance mode, right-click the host and select the Enter Maintenance Mode option: