A VTPv3 Primary server is the only switch in a VTP domain that is allowed to make configuration changes to the VLAN database to propagate out to the network. VTPv3 servers and clients will only exchange VLAN database information if they agree on the domain name and the identity of the Primary Server (MAC Address).
Other switches in the VTP domain who are configured to be servers are classified as secondary servers and cannot make configuration changes to the VLAN database. They are only there to serve as backup servers to the primary in case the existing primary server is demoted for any reason.
There are cases where there may be a conflict between switches on what each believes to be the primary server. Conflicting switches do not synchronize their VLAN databases, this can happen when a primary server is disconnected and another server is promoted. Then the original primary server is reconnected and comes back online, client switches then begin having a conflict as a result.
Correcting this simply means demoting one of the primary servers and allowing the other to take over as the only primary server in the VTP domain.