vSphere High Availability (HA) provides easy to use and cost-effective high availability for virtual machines and applications running in virtual machines. HA is part of Cluster and can be enabled from Cluster settings at the time of Cluster creation or later.
HA is a very important and useful feature of vSphere to keep virtual machines up and running with minimum downtime. HA keep eye on all hosts in the cluster and when any host failure happens HA, restarts affected VMs on another host in the cluster.