When network problem happens, they can interfere with communication between cluster nodes. A small set of nodes might be able to communicate together across a functioning part of a network, but might not be able to communicate with a different set of nodes in another part of the network. If this happens, at least one of the sets of nodes must stop running as a cluster.
To prevent the things that are caused by a split in the cluster, the cluster software requires that any set of nodes running as a cluster must use a voting algorithm to determine whether, at a given time, that set has quorum.
Because a given cluster has a specific set of nodes and a specific quorum configuration, the cluster will know how many votes constitutes a majority. If the number fall below the majority, the cluster stops running. The nodes will still listen for the presence of other nodes, in case another node appears again on the network, but the nodes will not begin to function as a cluster until the quorum exists again.
So,for example in a five node cluster that is using a node majority, what happens if nodes 1, 2, and 3 can communicate with each other but not with nodes 4 and 5? Nodes 1, 2, and 3 constitute a majority, and they continue running as a cluster.
Nodes 4 and 5 are a minority and stop running as a cluster, which prevents the problems of a split situation. If node 3 loses communication with other nodes, all nodes stop running as a cluster. All functioning nodes will continue to listen for communication, so that when the network begins working again, the cluster can form and begin to run.