When you want to have high availability in zookeeper server you use multiple zookeeper servers to create an ensemble. Basically, Zookeeper has a master-slave architecture. In an ensemble, there will be one master and the rest will be the slaves. If the master fails one of the salves will act as a master.
The sequence in which a master is called a quorum. When you create an ensemble, zookeeper internally creates a sequence ID for the slave servers. When the main master fails, it will check the next sequence ID to create a new master. This concept of quorum also used while creating nodes in zookeeper.