Failback is the process of restoring operations to a primary machine or facility after they have been shifted to a secondary machine or facility during failover.
During a site-wide failover, I/O (input/output) and its processes are shifted from a primary location to a temporary disaster recovery (DR) location. The process temporarily halts I/O and restarts it from a remote location. Any data copying and mirroring activity from primary location to the secondary location will be suspended and applications and I/O will be brought up from the secondary location. Any changes to data are tracked so that the original location can be restored by replicating the data between the start and end of the DR event. During the actual failback, data is re-synchronized -- halting I/O and application activity once again -- before operations are restored to the original location.