The cluster overcommit monitor indicates whether or not the host cluster is currently in an overcommitted state.
Based on the user defined cluster reserve, if a host cluster cannot withstand the specified number of node failures and still keep all of the virtual machines running, then the cluster is placed in an overcommitted state and the state of this monitor is unhealthy.
To return a Host Cluster to a non-overcommitted state, you may:
Reconfigure existing virtual machines
Modify the cluster reserve
Add additional hardware to the cluster