Hyper-V Replica provides protection to VMs by tracking and replicating changes to the virtual hard disks (VHDs) of the VM. Hyper-V Replica runs 24 hours, 365 days in a year; for any VM that has been enabled for replication it ensures that the data on the primary site and the Replica site are kept as closely in sync as supported.
Thus in a rainy day scenario, when HVR determines that the replica VM can no longer be kept in sync with the primary by applying the replicated changes then resynchronization is required. Resynchronization (or Resync) is the process of re-establishing the baseline – by ensuring that the primary and replica VHDs have exactly the same data stored.
This resynchronization cann happen when: