https://blogs.technet.microsoft.com/virtualization/2013/05/10/resynchronization-of-virtual-machines-in-hyper-v-replica/
What is resynchronization and why is it needed?
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.
To begin with, Hyper-V Replica (HVR) requires that the data on the virtual hard disks (VHDs) of the primary and replica VMs be the same. This is achieved through the process of initial replication, and establishes a baseline on which replicated changes can be applied. However, due to factors beyond the control of the administrator – such as faulty hardware and OS bugchecks – it is possible that the primary and Replica VMs are not in sync.
Thus in a rainy day scenario (details in following section), 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.