Configuring the Quorum in a Failover Cluster
In simple terms, the quorum for a cluster is the number of elements that must be online for that cluster to continue running.
In effect, each element can cast one "vote" to determine whether the cluster continues running.
The voting elements are nodes or, in some cases, a disk witness or file share witness.
Each voting element (with the exception of a file share witness) contains a copy of the cluster configuration, and the Cluster service works to keep all copies synchronized at all times.