A cluster host is a host that is configured to take over the role of another host server within a cluster. This practice eliminates the danger inherent in a single point of failure, and makes more efficient use of network resources.
A cluster host is a host machine that is able to support the various guest operating systems on another physical machine. So, when a physical machine goes down for maintenance or reboot, the cluster host should be configured in a manner that would allow it to support all of the original server's guest operating systems. This is all transparent to the end user, and it eliminates a possible single point of failure.